Material and device properties modification by electrochemical charge injection in the absence of contacting electrolyte for either local spatial or final states

ABSTRACT

In some embodiments, the present invention is directed to processes for the combination of injecting charge in a material electrochemically via non-faradaic (double-layer) charging, and retaining this charge and associated desirable properties changes when the electrolyte is removed. The present invention is also directed to compositions and applications using material property changes that are induced electrochemically by double-layer charging and retained during subsequent electrolyte removal. In some embodiments, the present invention provides reversible processes for electrochemically injecting charge into material that is not in direct contact with an electrolyte. Additionally, in some embodiments, the present invention is directed to devices and other material applications that use properties changes resulting from reversible electrochemical charge injection in the absence of an electrolyte.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 US National Phase of PCT/US2005/007084 filedon 4 Mar. 2005, which claims priority to U.S. Provisional ApplicationSer. No. 60/550,289 filed on 5 Mar. 2004.

This invention was made with partial Government support under ContractNo. MDA972-02-C-0044 awarded by the Defense Advanced Research ProjectsAgency. The Government has certain rights in this invention.

FIELD OF THE INVENTION

This invention relates to tuning the electronic, magnetic, and opticalproperties of materials and devices by non-faradaic electrochemicalcharge injection, wherein such tuned properties are developed either inthe presence or absence of contacting electrolyte, and necessarilymaintained in the absence of directly contacting electrolyte.

BACKGROUND OF THE INVENTION

It is well known that charge injection can change the magnetic,electronic and optical properties of materials. Prior-art methods forchanging these material properties by charge injection either (1)involve electrostatic gate-based charge injection across a dielectric(so charge injection is limited by dielectric breakdown), (2) use anelectrolyte that contacts the transformed material (thereby limitingdevice applicability), or (3) use dopant intercalation (thereby limitingapplicable materials and providing problematic structural changes).These three methods are called dielectric-based charge injection,non-faradaic electrochemical charge injection, and faradaicelectrochemical charge injection, respectively.

Dielectric-based charge injection is used for field effect transistor(FET) devices that are critical for both today's electronic circuits andthose proposed for the future. In these FET transistor devices, currentis carried through a semiconductor channel between source and drainelectrodes. The current through this semiconductor (channel) iscontrolled by charge injection into the semiconductor channel byapplication of a voltage between the source electrode and the gateelectrode, which is separated from the semiconductor channel by adielectric. This charge injection is that of an ordinary dielectriccapacitor, so the amount of charge injection that can be achieved islimited by the breakdown strength of the dielectric. While enormouslyuseful for submicroscopic electronic devices, this dielectric-basedcharge injection is unsuitable for macroscopic charge injection inmaterials having macroscopic external dimensions. X. Xi et al. (AppliedPhysics Letters 59 3470 (1991)) have demonstrated dielectric-basedswitching of the superconducting transition temperature (T_(c)) of filmsof YBa₂Cu₃O_(7−x) over a 2 K range. The achieved resistance modulationin the normal state can be as much as 20% in the normal state and 1500%near T_(c).

Using a similar method of dielectric-based charge injection in theoxygen deficient YBa₂Cu₃O_(7−x) superconductor, J. Mannhart et al.(Applied Physics Letters 62, 630 (1993)) demonstrated that T_(c) can bechanged by up to 10 K. However, the dielectric-based method of T_(c)switching used by Xi et al. and by J. Mannhart et al. is not applicablefor a macroscopically thick superconducting material. In addition, J. A.Misewich et al. (Science 300, 783-786 (2003)) have used dielectric-basedcharge injection to make an electrically driven light source from asingle nanotube. Also, Y. S. Choi et al. (Diamond and Related Materials10, 1705-1708 (2001)) have used under-gate dielectric-based chargeinjection to modulate electron emission for field emission displays, butfind disadvantage in this application as a result of field-inducedelectron beam spreading and restrictions on the anode voltage.

Non-faradaic electrochemical charge injection uses nanostructuredmaterials having very high surface area and is applicable for materialsranging from nanoscale materials to bulk materials. However, unless thematerial is a metal or metal oxide catalyst, the electrolyte is aceramic held at high temperatures (P. E. Tsiakaris, et al., Solid StateIonics 152-153, 721-726 (2002)) prior-art technologies teach that thischarge injection can only be accomplished by developing and maintainingcontact of the electrolyte with regions of the material where chargeinjection is desired, which for macroscopic nanoporous materialsincludes internal surfaces. In other words, the prior art teaches thatnon-faradaic electrochemical charge injection into non-catalyticmaterials generally requires maintained contact of that material withthe electrolyte. This non-faradaic electrochemical charge injection hasbeen used to provide electrochemical electromechanical actuators(artificial muscles, see R. H. Baughman et al., Science 284, 1340(1999), R. H. Baughman et al., U.S. Pat. No. 6,555,945) andliquid-ion-gated FETs (field-effect transistors, see M. Krüger, AppliedPhysics Letters 78, 1291-1293 (2001)). However, the maintained contactbetween the electrode (including both internal and external surfaces)and the electrolyte limits applicability of prior-art methods ofnon-faradaic electrochemical charge injection. For example, thesurrounding electrolyte for the above described liquid-ion-gated FETslimits their applicability for gas state sensing—since a sensed gas mustfirst dissolve in the electrolyte before it can be detected, whichdecreases both device response rate and sensitivity and limits detectioncapabilities to gases that can significantly dissolve in theelectrolyte. In addition, non-faradaic electrochemical charge injectionprovides the basis for supercapacitors having much larger charge storagecapabilities than ordinary dielectric supercapacitors. In the prior art(K. H. An et al., Adv. Funct. Mater. 11, 387 (2001) and C. Niu et al.,Appl. Phys. Lett. 70, 1480 (1997)) these supercapacitors are kept in thecharged state as a result of maintained contact between thenanostructured electrodes and the electrolyte. Since the electrolyteprovides mechanisms for self-discharge, long term energy storage in sucha supercapacitor is not possible. Also, the possibility of chargingnon-faradaic supercapacitors, removing the electrolyte, and then storingenergy in the dry-state supercapacitors has heretofore not beenconceived.

Faradaic electrochemical charge injection involves the intercalation ofions into a solid electronically conducting electrode material. Thismethod is limited to the types of materials that can incorporate dopantby a reversible process, preferably at room temperature. For example,elemental metals and metal alloys cannot undergo charge injection bythis method. Similarly, this method of charge injection is not useablefor non-porous materials having three-dimensional covalent bonding.Also, substantial dopant intercalation fundamentally changes thestructure of the material and can introduce gross structural defects. Asa consequence, de-doping does not completely return the material to theoriginal state. Nevertheless, the faradaic electrochemical method ofcharge injection has great value, as indicated by the year 2000 award ofa Nobel prize for the discovery that dopant intercalation (eitherchemically or electrochemically) into semiconducting conjugated polymerscan convert these semiconductors into metallic conductors. Faradaicelectrochemical doping (for conducting polymers and other materials) isused for both primary and rechargeable batteries (Y. Gofer et al.,Applied Physics Letters 71, 1582-1584 (1997)), conducting polymeractuators (R. H. Baughman, Synthetic Metals 78, 339 (1996)),electrochromic displays (W. Lu et al., Synthetic Metals 135-136, 139-140(2003)), the control of membrane ion permeability (P. Burgmayer and R.W. Murray, J. Phys. Chem. 88, 2515-2521 (1984)), the release of drugsand other biochemically active agents (H. Shinohara et al., ChemistryLetters, 179-182 (1985), L. L. Miller at al., U.S. Pat. No. 4,585,652),and electrochemical light emitting displays (G. Yu at al., Science 270,1789-1791 (1995)). For such devices, dramatic structural changes aretypically associated with dopant intercalation, and these charges arenot fully reversed on de-intercalation—which limits cycle life. Therequired dopant insertion and de-insertion processes (calledintercalation and de-intercalation) result in slow device response,short cycle life, hysteresis (leading to low energy conversionefficiencies), and a device response that depends on both rate anddevice history.

The embodiments of the present invention eliminates key problems ofthese prior-art technologies, by showing that non-faradiacelectrochemical charge injection can be maintained, and even developed,at room temperature for regions of the electrode that are not in directphysical contact with the electrolyte. The present discoveries enablematerials and device applications that would not be possible, or wouldbe less advantageous, in the presence of locally contacting electrolyte.

BRIEF DESCRIPTION OF THE INVENTION

A first object of the invention is to provide processes for thecombination of injecting charge in a material electrochemically vianon-faradaic (double-layer) charging and retaining this charge andassociated desirable properties changes when the electrolyte is removed.

A second object is to provide compositions and applications usingmaterial property changes that are induced electrochemically bydouble-layer charging and retained during subsequent electrolyteremoval.

A third object is to provide reversible processes for electrochemicallyinjecting charge into material that is not in direct contact with anelectrolyte.

A fourth object is to provide devices and other material applicationsthat use properties changes resulting from reversible electrochemicalcharge injection in the absence of an electrolyte. Examples of suchapplication of charge injection and associated magnetic, optical, andelectronic properties changes are for optically transparent electronicconductors; spintronic devices; information storage devices;nanostructured magnets; chemical and mechanical sensors;electromechanical actuators; the control of thermal and electricalenergy transport; the tuning of surface energy and friction; theswitching, phase shift, and attenuation of electromagnetic radiation;tuning magnetoresistive materials; and drug delivery.

Invention embodiments include processes for improving materialproperties by non-faradaic charge injection and retaining these switchedproperties in the absence of electrolyte that contacts charge-switchedelectrode regions.

More specifically, in some embodiments, the present invention providesfor processes whose overall effect is to provide, retain and employcharge injection to substantially change the properties of a material A,material A being either a largely electrolyte-free porous materialregion, or a particle, the process comprising the steps of: (a)immersing material A into an electrolyte E; (b) providing an ionconducting and substantially electronically insulating continuous pathbetween material A and a counter electrode material B; (c) applying apotential between material A and the counter electrode B for sufficienttime that the desired charge is injected into material A; and (d)substantially removing the electrolyte E from contact with material A,wherein both material A and counter electrode B have an electronicallyconducting charged or uncharged state and material A has an achievablecapacitance for non-faradaic charging of above about 0.1 F/g.

Compositions of matter resulting from the above processes are alsoprovided in the invention embodiments. An example is a composition ofmatter containing non-faradaically injected charge and substantially noelectrolyte that maintains, in a suitable environment, a potential thatdeviates from the potential of zero charge by at least 0.1 volt.

Devices that utilize non-faradaic charge injection in the absence oflocally contacting electrolyte are also provided in inventionembodiments. These devices, having a tunable response, comprise: (a) ananostructured electrode component C of a first electrochemicalelectrode and an electrode component D of a second electrochemicalelectrode; (b) an ionically conducting material that is substantiallyelectronically non-conducting that connects said first and said secondelectrochemical electrodes; and (c) a means of providing a voltagebetween said first and said second electrochemical electrodes, whereinthe electrode component C is not in direct contact with an electrolyte,the electrode component C has an achievable capacitance of above about0.1 F/g for substantially non-faradaic charging, and wherein propertieschanges of the electrode component C in response to injected charge areused to achieve device performance. In some embodiments it isadvantageous if said first electrochemical electrode and said secondelectrochemical electrode are both porous electrodes having acapacitance of at least about 0.1 F/g, and wherein said ionicallyconducting material that is substantially electronically non-conductingat least partially penetrates both said first and said secondelectrochemical electrodes.

The foregoing has outlined rather broadly the features of the presentinvention in order that the detailed description of the invention thatfollows may be better understood. Additional features and advantages ofthe invention will be described hereinafter which form the subject ofthe claims of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 shows the observed tunability of four-point electricalconductivity as a function of applied potential (versus Ag/Ag+) for asheet of single-wall carbon nanotubes immersed in an organic electrolyte(0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile). Thedifferent curves are for three successive cycles of electrochemicalpotential change (using squares, circles, and triangles for successivecycles).

FIG. 2 shows the dependence of four-point electrical conductivity uponthe amount of injected charge (per carbon) for the nanotube sheet usedfor the FIG. 1 experiment (black data points are for experiments using0.1 M tetrabutylammonium hexafluorophosphate/acetonitrile electrolyteand near-white data points are for related measurements using 1 Maqueous NaCl electrolyte). The origin of the charge scale is arbitrary.

FIG. 3 shows measured cyclic voltammetry for the SWNT sheet of FIGS. 1and 2 when immersed in tetrabutylammonium hexafluorophosphateelectrolyte. These results show that the charging is predominatelynon-faradaic by double-layer charge injection.

FIG. 4 shows that the dramatic hole-injection-induced increase inelectrical conductivity of the nanotube sheet in FIGS. 1 and 2 islargely retained when the hole-injected electrode is dried in a flowingdry nitrogen atmosphere to remove the electrolyte. The insert to thisfigure shows results for the initial four-hour period on an expandedtime scale.

FIG. 5 shows the retention of conductivity enhancement when the nanotubesheet is removed from the electrolyte and held in air for theinvestigated five-day period.

FIG. 6 depicts cyclic voltammetry measurements (20 mV/sec using 1 Maqueous NaCl electrolyte) for nanostructured platinum electrodes,showing that charging is non-faradaic by double-layer charge injection.

FIG. 7 shows potential measurements before and after removal of thenanostructured Pt electrodes of FIG. 6 from the 1 M NaCl aqueouselectrolyte (and subsequent reimmersion into the electrolyte). Themeasured electrode potentials of both positive and negative electrodeindicate that non-faradaically injected charge is partially retainedeven when the electrodes are removed from the electrolyte.

FIG. 8 shows a scanning electron microscope (SEM) picture of ananostructured inverse-opal carbon electrode that was found to storecharge without the need for a surrounding electrolyte.

FIG. 9 uses carbon nanotube electrodes to schematically illustrate ameans used in invention embodiments for injecting charge into regions ofa high-surface-area electrode that is not directly contacted withelectrolyte.

FIG. 10 schematically illustrates a prior-art device technology forusing electrochemical double-layer charge injection to switch theconductivity of a semiconducting channel material. This device uses theprior art technology of liquid-ion gating, which means that the gate isimmersed in a liquid electrolyte that provides the necessary ions asliquid state species.

FIG. 11 schematically illustrates a first electrochemical transistordevice of the present invention that does not use liquid-ion gating,which could be used for information storage, electronic switching, orgas sensing.

FIG. 12 schematically illustrates a second electrochemical transistordevice of the present invention that does not use liquid-ion gating,which could be used for information storage, electronic switching, orgas sensing.

FIG. 13 schematically illustrates an optical gas sensor, based on thesurface-enhanced Raman effect, that uses electrochemically controlledcharge injection in a metallo-dielectric photonic crystal to optimizesensitivity and species selectivity.

FIG. 14 schematically illustrates a fuel cell of invention embodiments.Unlike the case of prior-art fuel cells, the fuel cell redox reactionspredominately occur in the gas phase (on surfaces of carbon nanotubesthat are exposed to the H₂ and O₂ gas) without directly contactingelectrolyte.

FIG. 15 schematically illustrates a tunable nanotube device in whichtunability results from an electrochemically-induced insertion of ionsinside a nanotube, and insertion of associated counter electroniccharges onto the nanotube.

FIG. 16 schematically illustrates a supercapacitor of inventionembodiments that can be charged, drained of electrolyte, partially orcompletely evacuated then reactivated for subsequent discharge in aremote location by refilling with electrolyte. In the illustrated case(showing a cross-section of the device normal to the supercapacitorelectrode sheets), the electrolyte for device refill is carried in acompartment of the device. In an alternative invention embodiment, theelectrolyte (which can be salt water) is injected into thesupercapacitor device from an electrolyte container that is separatefrom the device.

FIG. 17 schematically illustrates a filtration device that usesnon-faradaic charging to dynamically and selectively control thetransport of material through a membrane having discreet pores.

FIG. 18 schematically illustrates an electromechanical actuator deviceof invention embodiments that can provide much larger actuation strainsthan any prior art device of any type. This device uses a carbonmultiwalled nanotube (MWNT) which telescopes outward in order todecrease free energy by increasing the surface area available for chargeinjection.

FIG. 19 schematically illustrates an electrochemically-gated device thatcan be used for atomic probe imaging and electron emission.

FIG. 20 schematically pictures a device that operates like the device ofFIG. 19, except that the nanotube probe tip base makes electricalcontact to the nanotube. Such design facilitates device construction.

FIG. 21 schematically illustrates a device in which a high-surface-areananostructured material functions as an electrochemically-gated ion beamsource, that can be modulated at will by the application of low voltagepotential between electrochemically-active electrodes.

FIG. 22 schematically illustrates a cross-sectional view of a device inwhich individual nanoscale electrodes (pixels) are electrochemicallycharged either positively or negatively using a focused electron beam.In contrast with the case of other device illustrations, electrochemicalcharging does not require a wire lead to the electrode, so extremelysmall pixels can be conveniently addressed within a densely packed pixelarray.

FIG. 23 schematically illustrates one type of device of inventionembodiments that provides tunable thermal conductivity.

FIG. 24 illustrates an invention embodiment in which predominatelynon-faradaic charge injection is used to optimize the figure of merit(ZT) of thermoelectric elements interconnected by electrolyte.

FIG. 25 illustrates a device embodiment of the present invention whereelectromagnetic radiation propagates within the device either at leastapproximately parallel to the electrode layers or predominately alongthe lengths of element 2500 (and like elements), along the lengths ofelement 2502 (and like elements), or along both of these types ofelements (in contrast with the possibly approximately perpendicularpropagation of the electromagnetic radiation for the device of FIG. 13).

FIG. 26 schematically illustrates an EBIG (Electrolyte-Bare Ion Gated)device that provides electroluminescence by using a semiconductingnanostructured material—in this case a semiconducting carbon nanotube.

FIG. 27 schematically illustrates another EBIG that is light emitting,wherein the light-emitting element is a semiconducting inverse-opalphotonic crystal.

FIG. 28 schematically illustrates a back-gated electrolyte-bare ion gate(EBIG) field effect transistor that has an air gap.

FIG. 29 illustrates a device embodiment of the present invention,wherein the advantageous combination of non-faradaic electrochemical andgas-gap-based electrostatic charge injection for a chemical sensor thatcan replace conventional Chem-FETs, is depicted. This illustratedembodiment depicts a Chem-FET sensor device that is at the same time aEBIG sensor.

DETAILED DESCRIPTION OF THE INVENTION

Invention embodiments are directed to processes, materials, and devicesthat utilize non-faradaically injected charge and associated changes ineither magnetic, electronic, optical, or chemical properties, whereinthe non-faradaically injected charge is retained in the absence ofdirect or maintained contact with the electrolyte.

Differentiation between faradaic and non-faradaic charging processes isimportant for understanding the advantages and novelty of theseinvention embodiments. Significant amounts of electronic charge can beinjected stored) in an electrode typically only if counter ions ofopposite charge are available in close proximity to the electroniccharge injected into the electrode. These counter ions can compensatethe electrostatic repulsion of the electronic charges on the electrode,thereby enabling the electronic charge injection process to proceed tohigh levels. A faradaic process for an electrode material is one inwhich electronic charge injected into the electrode is predominatelycompensated by ions that are inserted (i.e., intercalated) into thevolume of the charge-injected component of the electrode. A non-faradaicprocess is one in which compensation of electronic charge on theelectrodes is by ions that do not enter the solid volume of thecharge-injected component of the electrode. A well-known example of anon-faradaic process is one in which the counter ions (to the electroniccharge on the electrodes) are located in the electrolyte (within theso-called charge double layer). Evidence of faradaic charging processesis provided by the existence of well-defined peaks in cyclicvoltamograms (current-versus-potential plots at constant potential scanrates). In contrast, the current in a cyclic voltamogram depends onlyweakly on potential in a potential range where electrode charging isnon-faradaic and the electrolyte is neither reduced nor oxidized.Correspondingly, for the purposes of this invention, the existence ofpotential range, where electrode potential increases substantiallylinearly with injected charge under some charging conditions, issufficient (but not necessary) evidence for predominately non-faradaiccharging in this potential range for these charging conditions. If ionscorresponding to injected electronic charges are predominately locatedon or near the closest surface of an electrode material, including theinternal surfaces arising from material porosity and the interior volumeof hollow fibers, the charging process is herein defined asnon-faradaic, independent of the shape of cyclic voltammetry curves. Thepore size in a charge injected electrode structure can be at any sizethat is larger than that of the incorporated ions (together withpossible solvating species), since this is the situation needed in orderto avoid harmful dimensional charges as a result of the volume of theion (together with possible solvating species). Charging by ionincorporation within a nanotube, such as a carbon nanotube or a carbonscroll is specifically designated as being non-faradaic for the purposesof this invention.

Prior-art understanding of non-faradaic charge injection is that theelectrode capacitance (and therefore the non-faradaic charge injectioncapability) for an electrode is given by the following equation:C_(e)=A_(e)C_(s). In this equation, C_(s) is the capacitance per unitsurface area of the conducting component of the electrode when thissurface area is in contact with electrolyte. A_(e) is the total surfacearea of the electronically conducting component of an electrode that isin contact with a suitably thick layer of electrolyte. This electrodesurface area includes all electrolyte-coated internal and externalsurface area of the electronically conducting component of theelectrode. Ignoring the potential dependence of C_(s), the amount ofinjected electrode charge is q_(e)=(V−V_(o))C_(e), where V_(o) is thepotential at which there is zero electronic charge injected in theelectrode and V is the potential applied to the electrode, measured withrespect to the same reference electrode as V_(o). Applicants havediscovered that these equations are not generally valid when only partof a nanostructured electrode is in contact with the electrolyte. Theprocesses that lead to violation of these equations are the basis forcertain embodiments of this invention.

More specifically, Applicants have discovered that dopant ions (togetherin some instances with solvating species from the electrolyte) canmigrate at room temperature from the electrolyte (solid-state or liquid)to regions of the electrode that are not coated with electrolyte. Thisdopant ion migration enables electronic charge to be injected in regionsof the electrode that are not in contact with the electrolyte, therebyincreasing the injected charge (q_(e)) above the values given by theequation given in the preceding paragraph. Most important for deviceapplications, such as field emission displays, Applicants' discoverymeans that electrochemical double-layer charge injection can be used toinject charge in regions of the electrode that are in a gas or vacuum,rather than in an electrolyte.

This enabling and unexpected discovery resulted from Applicantssurprising observation that charge can be non-faradaically injected intoa nanoporous electrode, and then retained in this electrode when thiselectrode is removed from a volatile electrolyte and dried by pumping indynamic vacuum. This observation means that the optical, magnetic, andelectronic properties of an electrode material can be tuned, and thatthese tuned properties can be substantially maintained for materialsapplications in ambient environments, where the problematic electrolyteis absent.

Also generally important for invention embodiments, the stabilityobserved (in the absence of electrolyte) for injected charge on surfacesof nanostructured materials (internal and external) indicates that onecan continuously vary the degree of charge injection in nanostructuredmaterial elements that are not in direct contact with the electrolyteand maintain this injected charge when the power source is disconnected.This stability and the continuous tunability of charge injection in theabsence of direct electrolyte contact enables many of the materials anddevice applications in the device embodiments. These embodiments requirethat the continuous tuned material elements are electronically connectedto an electrode, so that the electronic part of charge injection canoccur. Additionally, these embodiments require that ion conductionpath(s) exist or can self form to the continuously tuned elements—whichneed not be in direct contact with the electrolyte.

Processes of the present invention generally require at least twoelectrodes, called working and counter electrodes. For the cases wherethe desired charge-injection-produced properties changes are for onlyone of these electrodes, the working electrode is defined as thiselectrode. In addition, one can optionally employ a reference electrode,whose function is to place the potentials of the working and counterelectrodes on an absolute scale. The working electrode should typicallyhave a capacitance of at least 0.1 F/g when fully immersed inelectrolyte. More typically, the capacitance of the working electrodefully immersed in the electrolyte should exceed 1 F/g. Most typically,the capacitance of the working electrode should exceed 10 F/g when fullyimmersed in electrolyte. The specific capacitance of an electrode can bederived using the conventional method from the dependence of gravimetriccurrent on the scan rate of electrode potential (versus a referencepotential). (See J. Li et al., J. Phys. Chem. B. 106, 9299-9305 (2002)for a description of this method.) The above-mentioned specificcapacitances for the typical, more typical, and most typical inventionembodiments are the maximum values that can be measured within thestability range of the electrolyte.

Before discussing particular embodiments of the present invention, somesurprising observations that provide basis for these embodiments will bedescribed. These discoveries are further elaborated in the examplessection. Following this, elaboration will be provided on (1) specializedinvention embodiments and (2) material components and processing methodsfor the practice of these embodiments.

The results depicted in FIGS. 1 and 2 show that a potential change andcorresponding electrode charge injection can dramatically change theproperties of a nanostructured electrode material. FIG. 1 shows thepresently observed continuous tunability of four-point electricalconductivity as a function of applied potential (versus Ag/Ag+) for asheet of single-wall carbon nanotubes (SWNTs) immersed in an organicelectrolyte (0.1 M tetrabutylammonium hexafluorophosphate inacetonitrile). These results demonstrate that the electricalconductivity of the nanotube sheet can be increased by about an order ofmagnitude by electrochemical charge injection. There is slighthysteresis evident for the curves in FIG. 1, with the conductivity a onthe extreme left side of the potential minimum being slightly higher forhole injection (increasingly positive applied potential) and theconductivity slightly lower on the extreme right side of the minimum forelectron injection (increasingly negative applied potentials). Thedifferent curves are for three successive cycles (using squares (101),circles (102), and triangles (103) for successive cycles). The densityof these nanotube sheets is about 0.3 g/cm³, versus the density of about1.3 g/cm³ for densely packed nanotubes having close to the observedaverage nanotube diameter. Hence, the void volume in these nanotubesheets is about 76.9 volume percent. This high void volume, and thecorrespondingly high accessible surface area, is generally important forachieving high degrees of non-faradaic charge injection at modestapplied potentials. Supporting this conclusion, the measured BET surfacearea determined from nitrogen gas adsorption for these nanotube sheetsis approximately 300 m²/g.

FIG. 2 shows the dependence of four-point electrical conductivity uponthe amount of injected charge for both the above experiment with 0.1 Mtetrabutylammonium hexafluorophosphate/acetonitrile electrolyte (blackdata points (201)) and for other experiments using 1 M NaCl electrolyte(near-white data points (202)). Although not indicated here in this plotof conductivity versus charge (because charge measurements are notreliable for potentials exceeding the redox stability of aqueouselectrolyte), reversible conductivity increases from about 100 S/cm toabout 1000 S/cm were also observed at positive potentials forexperiments using the 1 M aqueous NaCl electrolyte. The origin of thecharge scale is arbitrary. The charge per carbon at the minimum inelectrical conductivity can be used to place origin of the charge axis,since theory suggests that conductivity should be minimized at thepotential of zero charge (pzc), where there is no charge on the carbonnanotube.

FIG. 3 shows measured cyclic voltammetry during charge injection for theabove SWNT sheet when immersed in the above tetrabutylammoniumhexafluorophosphate electrolyte. The absence of major peaks in thiscyclic voltammetry curve (measured versus Ag/Ag+) indicates thatcharging is predominately non-faradaic for this electrolyte andpotential range.

FIG. 4 shows that the dramatic increase in electrical conductivity ofthe nanotube sheet in FIGS. 1 and 2 (obtained by hole injection for thenanotube sheet and the organic electrolyte of these examples) is largelyretained when the hole injected electrode is dried in flowing drynitrogen atmosphere to remove the electrolyte. The insert to this figure(describing results over a four-day period) shows first a conductivityincrease and then a conductivity decrease during the first few hours ofthis four day study, which might be a result of volatilization of theacetonitrile component of the organic electrolyte. FIG. 5 shows theretention of conductivity enhancement when the nanotube sheet is removedfrom the electrolyte and held in air for the investigated five-dayperiod. These results show that the enhancement of electricalconductivity of the hole-doped nanotube sheet is relatively stable evenin atmospheric air. The electrical conductivity enhancement (andretained charge) degrades more rapidly in air for the hole-injectedelectrode than for the electron-injected electrode.

Applicants have found equally unexpected results when these roomtemperature measurement results were extended to generic nanostructuredmaterials, and, in particular, to a nanostructured metal—which has nopossibility of accommodating ions by faradaic processes (intercalation).These results are for a platinum electrode made by compaction of 30 nmdiameter Pt nanoparticles, using the method described by J. Weissmülleret al. in Science 300, 312 (2003). The cyclic voltammetry results inFIG. 6 (using 1 M aqueous NaCl electrolyte) show that these electrodesprovide the classical dependence of current on applied potential thatarises for double-layer charging. There are no current peaks due toFaradaic processes and the current at constant voltage scan rate (20mV/sec) varies little with potential. From plots of current versuspotential scan rate in 1 M aqueous NaCl electrolyte), the electrodecapacitance is about 14.5 F/g. The high volume fraction of void spaceobserved in these pellets (between 81.6 and 87.2 volume percent forcompaction pressures between 0.6 MPa and 2.1 MPa), together with thecorresponding high gravimetric surface area, explains the high degree ofnon-faradaic charge injection that results for modest applied potentialfor the nanostructured Pt electrode.

Perhaps most importantly, Applicants have found that the nanoporous Ptelectrode remains charged when disconnected from the power source andremoved from the electrolyte. Initial results indicating this stabilityare shown in FIG. 7. As shown in FIG. 7, the charge on the positiveelectrode (i.e., the hole-doped electrode) is retained when theelectrode is removed from the electrolyte, and then held in air.Indication of this retained charge is provided by reimmersion of thenanoporous hole-injected Pt electrode in the 1 molar NaCl electrolyte,and finding that the electrode potential is substantially unchanged.Just like the case for the carbon nanotube electrode, the potential ofthe negatively charged electrode is less stable than for the positivelycharged electrode, as indicated by the results shown in the lower partof FIG. 7.

Since it is possible that some electrolyte is still retained inside thepores of the Pt electrode during the experiment depicted in FIG. 7, thisexperiment was repeated using much longer time periods when theelectrodes are not in contact with the 1 M NaCl electrolyte, anddynamically pumping on the nanoporous Pt during this time period afterthe pellet electrodes have been disconnected from the power source andthe electrolyte was removed from the electrochemical cell. The electrodepotentials (versus Ag/AgCl) before and after this two day exposure ofthe electrodes to dynamic pumping were +0.58 V and +0.45 V for thehole-injected electrode and −0.04 V and +0.02 for the electron-injectedelectrode. The potential between the two electrodes changed from theinitial 0.62 V to a final 0.43 V after removal of the electrodes fromthe electrolyte and dynamically pumping on these electrodes for twodays.

To further evaluate the stability of charge storage, the time period inwhich the platinum pellets were exposed to dynamic vacuum was extendedto a week. After this, the nanoporous Pt electrodes were reimmersed inthe 1 M NaCl electrolyte to determine their charge state byelectrochemical potential measurements (naturally, without applying anyexternal potential). High charge storage was again indicated for thepositively charged Pt electrode (indicated by retention of a 0.28 Vpotential, versus Ag/AgCl, compared with the initial potential beforeelectrolyte removal of 0.33 V). The negatively charged electrode hadlower stability, as indicated by a potential change from the initial−0.68 V (before removal of the electrolyte and the week-long process ofdrying the electrode in dynamic vacuum) to a final potential on initialreimmersion into the electrolyte of −0.32 V.

Electron and hole injection in the nanostructured platinum samples doesnot significantly change electrical conductivity (in contrast with thecase for the nanostructured nanotube sheets), since this chargeinjection causes only a small fractional change in the already hightotal electron concentration. While it is known that charge injection ina liquid electrolyte can provide changes in unit cell volume, the priorart did not anticipate that these charge-induced volume changes can beretained in electrolyte-free materials (J. Weissmüller et al., Science300, 312 (2003) and R. H. Baughman, Science 300, 268-269 (2003)).

The implications of these results for practical applications ininvention embodiments are profound, since diverse properties (includingsuperconductivity, magnetism, and magneto-resistance) can be verysensitive to material volume.

Applicants have also shown experimentally that inverse opals (alsocalled inverse-lattice photonic crystals) of conducting materials areanother type of composition that can be used in some inventionembodiments. In these experiments, a carbon inverse opal was synthesizedby infiltrating phenolic resin into an ordinary SiO₂ opal, pyrolizingthis phenolic resin, and then removing the SiO₂ template material bydissolution in HF solution. A scanning electron microscope (SEM) imageof this carbon inverse opal is shown in FIG. 8. Experiments done in both1 M aqueous electrolyte and tetrabutyl ammoniumhexafluorophosphate/acetonitrile electrolyte show thatelectrochemically-injected charge is partially retained when theelectrodes are removed from the electrolyte and dried, and that thestability of injected charge is much higher than for the same electrodesimmersed in the electrolyte with the power source disconnected.

This retention of electrochemical double-layer-injected charge andcharge-injection-induced properties changes, when the charge-injectedmaterial is not in contact with the electrolyte, is more broadlyimportant for invention embodiments. Also important, Applicants havefound that dry-state-retained injected charge is highly mobile, asevidenced by Applicants' observation that the charge on dry negativelyand positively charged nanostructured electrodes, rapidly occurs whenthese electrodes are contacted in the dry state.

Processes, materials, and device applications stemming from theabove-described discovery of a means for electrochemically-injectingcharge in nanostructured materials and retaining this injected chargewhen the nanostructured material is not in contact with the electrolyte,is described next. These applications utilize Applicants' discoveriesthat non-faradaically injected charge and associated properties changesare retained in the absence of electrolyte. Use is also made ofApplicants' discovery that non-faradaically injected charge (ion andcorresponding electronic counter charge) of an electrolyte-freeelectrode can be highly mobile. Because of this mobility of the ions andelectronic charge, electronic charge and counter ions can be reversiblyand controllably electrochemically inject into nanostructured materialelements that are not placed in direct contact with an ion source (forexample, an electrolyte or intercalated material, such as a dopedconducting polymer).

Both here and elsewhere herein the terms “free of electrolyte,” “notdirectly contacted with electrolyte,” “not contacted by theelectrolyte,” “largely electrolyte free,” and like terms have a specificmeaning that is now defined. These terms apply for a material element αif any of the following cases applies: (1) material component α was atno point placed in physical contact with a bulk electrolyte compositionemployed for either processing, device fabrication, or device operation;(2) either anions, cations, and/or solvating species are present only inabout 10 nm or thinner surface layers on or within material component αor as salt crystals on or within material component α that cannot serveas an effective electrolyte under application conditions; (3)electrolyte-derived chemical species in materials component α that canbe almost entirely removed from the bulk electrolyte by pumping indynamic vacuum at room temperature are essentially stable in materialscomponent α under the same conditions; (4) the ratio of free anions tofree cations that are present in materials component α (i.e., those thatare not crystallized as salt crystals) is either lower than 0.9 or above1.1; and (5) major components in the utilized electrolyte aresubstantially absent in materials component α. Defining the meaning ofthese terms and like terms is pertinent, since the devices of inventionembodiments operate by the surface diffusion of ions from theelectrolyte and such ion diffusion can be accompanied by co-diffusion ofion-solvating species from the electrolyte.

First to be considered is a process of the present invention, whereinthe properties of a nanostructured material are tuned by non-faradaiccharge injection in an electrolyte, and wherein these properties changesand associated injected charge are retained when the electrode is freeof electrolyte. In perhaps the simplest embodiment of this process, thematerial to be processes by electrochemical charge injection is immersedinto a liquid electrolyte that is based on a solvent that can bevolatized (like water or acetonitrile). This material component is usedas the working electrode of an electrochemical cell that comprises thisworking electrode and a counter electrode. A reference electrode canalso be present in this electrochemical cell, since measurement of thepotential of this working electrode with respect to this referenceelectrode can be used to regulate the charge injection process. Thisnanostructured material, containing electrolyte in its pore structure,is charged by application of a potential difference between the workingand counter electrodes. Satisfactory completion of the desired degree ofcharge injection can be monitored by measuring either the total chargepassed through the electrochemical cell or the evolution of electrodepotential versus a reference electrode. After charge injection, thenanostructured electrode (or electrodes) in which charge injection isneeded can be disconnected from the power source and then dried orotherwise processed to remove liquid electrolyte (such as by washing inthe solvent component of the electrolyte, followed by drying). In orderto retain injected charge in the dried electrode, it is generallyimportant that the electrode material is not contacted with materialsthat can undergo redox reactions with charge on the electrode. Avoidingsuch degradative reactions is typically more difficult forelectron-injected electrodes than it is for positively injectedelectrodes, although this problem is decreased in difficulty when thedegree of charge injection in the electrode is not high.

A useful method for substantially eliminating charge degradation ofelectron-injected materials (by reaction with hole-donating impurities)is to enclose these materials in an environment that comprises “getter”substances that are easier to oxidize than the electron-injectedmaterial. Likewise, a useful method for substantially eliminating chargedegradation of hole-injected material (by reaction withelectron-donating impurities) is to enclose these materials in anenvironment that comprises getter substances that are easier to reducethan the hole-injected material. A host of materials can be used asgetter substances, such as Li metal or n-doped conducting organicpolymers for electron-injected materials or device components andheavily p-doped conducting polymers for hole-injected materials ordevice components. In some invention embodiments where neither increasesor decreases in charge injection are desirable, it is helpful if redoxactive components of the getter material do not contact thecharge-injected material. For this purpose, the getter material, andcomponents thereof, should ideally be immobile with respect to diffusionand volatilization at the normal operating temperature of the device orthe charge injected material. An important exception to this is the casewhere contact of the getter material is used to maintain theelectrochemical potential (and therefore the degree of charge injection)at a value set by the getter material. In such embodiments, the gettermaterial is likely to be an alkali metal, an alkali metal alloy, or adonor or acceptor intercalated material.

In some embodiments, processes of invention embodiments can be describedas providing, retaining, and employing charge injection to substantiallychange the properties of a largely electrolyte-free porous materialregion A. Such processes comprise the steps of: (a) placing anelectrolyte within the interior of A, (b) providing an ion conductingand substantially electronically-insulating continuous path between Aand a counter-electrode material B, (c) applying a potential between Aand counter electrode B for sufficient time that the desired charge isinjected into A, and (d) substantially removing the electrolyte fromcontact with A, wherein both A and B have an electronically conductingcharged or uncharged state and A has an achievable capacitance fornon-faradaic charging of above about 0.1 F/g. A material produced bythis process is referred to herein as an “Electrolyte-Bare Ion-Gated”material (EBIG material), since charge injected electrochemically bydouble-layer ion gating is retained and the electrolyte is substantiallyabsent in the final state.

The electrolytes used for process step (a) can be any of the manyaqueous and organic electrolytes later-described. While solid-stateelectrolytes can be employed, they are generally less desirable thanliquid electrolytes having electrolyte components that are easilyvolatilized in step (d) of the process. Electrolytes having high redoxstability (such as the below-listed ionic liquids) are especially usefulwhen high degrees of electrode charge injection are needed. Thecomposition of material A generally depends upon the intended use of thecharge-injection modified material, and these compositions for variousapplications will be later-elaborated. The composition of electrode B isrelatively unimportant unless the charging of this electrode isdesirably used for the production of material that is oppositely chargeinjected to that of electrode A (or unless specific properties changesare needed for electrode B in a device application). If the function ofelectrode B is merely to provide a counter-electrode to material A, thecapacitance of this electrode is typically at least twice that ofelectrode A, so that most of the applied potential between electrode Aand electrode B is applied across electrode A. The potential appliedduring step (c), the time evolution of this potential, and the durationof this potential depend upon the redox stability window of theelectrolyte and whether or not redox processes occurring in theelectrolyte will harmfully degrade either the electrolyte or theelectrodes. It is simplest to use either constant current or a constantinter-electrode potential between electrode A and electrode B. However,it is useful for maximizing rate for batch materials processing thatresistance compensation be used to control the inter-electrodepotential, in order to insure that both rapid charge injection occursand damage to either the electrodes or electrolyte due to excessiveapplied potentials is avoided. J. N. Barisci et al. (Journal of SmartMaterials and Structures 12, 549-555 (2003)) describe the use ofresistance compensation for another use (electrochemicalelectromechanical actuators), and the concepts described by theseauthors are applicable for the present application.

One measure of the degree of charge injection is the magnitude of thedeviation of electrode potential from the potential of zero charge. Thisdeviation for an electrode element that is not substantially contactedwith electrolyte is typically above about 0.1 V for selected inventionembodiments. More typically, this deviation should be above about 0.4 Vfor these invention embodiments.

The process step of charging an electrode immersed in an electrolyte canbe accomplished under conditions that optimize the degree ofinfiltration of the electrolyte within the pore space of theelectrolyte, since this optimizes the degree of charge injectionachieved at a particular potential (via increasing the realizedcapacitance of the electrode material in the electrolyte). For thepurpose of increasing electrolyte infiltration by improving electrolytewetting of the electrolyte, a series of potentials can be applied whoseeffect is to remove charge from the electrode and then to re-injectcharge into the electrode. For example, a constant voltage scan rate canbe used to cycle the applied electrode between minimum and maximumpotentials one or more times, so as to leave the electrode in a desiredstate at the end of the cycling process. The point is that, whilecharging can be usefully accomplished for regions of the electrode thatare not directly contacted by the electrolyte, the degree of chargingachieved at a particular potential is generally less than that achievedif the electrode is in contact with the electrolyte. For electrodes thatare difficult to infiltrate with electrolyte, and situations wheremultiple cycling does not undesirably increase process cost or degradeelectrode structure, this process of cycling, for the purpose ofenhancing charge injection, can typically be accomplished at least aboutthree times after the initial charge injection. This electrochemicalcycling process includes the steps of applying a series of potentialswhose effect is to remove charge from the electrode and then tore-inject charge into the electrode, so as to thereby increase therealized gravimetric capacitance of the electrode.

Processes of invention embodiments for employing charge injection tosubstantially change the properties of a largely electrolyte-free porousmaterial can be applied to various forms of these materials, such assheets, fibers, and powders. Various methods can be used for convenientnon-faradaic electrochemical charge injection into powders, such asnanofiber powders. A first method is to assemble these powders into asolid electrode form, such as a sheet, use this sheet as an electrodefor predominantly non-faradaic charge injection, and then to break thesesheets into powders.

A second method is to disperse the powder in electrolyte, in at leastone compartment of a separated-compartment electrochemical cell (wherethe anode compartment and cathode compartments are separated by eitheran ion conducting membrane or a porous frit). Agitation of thedispersion of powder in the electrochemical cell (such as by stirring orother electrolyte flow process) brings the powder particles or fiberinto intermittent contact with the electrode of this compartment,thereby permitting non-faradaic charging of the powder. The oppositecompartment of this electrochemical cell can have aconventionally-shaped electrode (such as a sheet) and can, optionally,also contain an agitated electrolyte-dispersed powder that can be eitherthe same or different from the powder in the other cell compartment. Anadvantage of this arrangement is that two powder samples can be charged(albeit with opposite charge) during the charging step of the process.

A third method is to use a moving conducting belt to carry the powderinto and out of the electrochemical cell, wherein this moving belt ispart of the electrode. The opposite electrode (typically in a separateelectrode compartment) can either include a conventionally configuredelectrode (such as a sheet), or can contain the same type as thebelt-electrode powder delivery system as for the other mentionedelectrode.

A fourth method is to disperse the powder in the electrolyte of anelectrode compartment, and to configure this compartment so that theeffect of gravity is to provide contact between the powder and theelectrode of this compartment (which is typically a planar electrode).In this method the electrolyte is selected to be either substantiallyheavier or substantially lighter than the powder. In the former case theelectrode of the compartment is in the lower most region of theelectrolyte compartment, and in the latter case the electrode is in theupper most region of this compartment.

After predominately non-faradaic charge injection, the charge injectedparticles can be removed from the electrolyte and optionally washed anddried without completely losing the injected charge. Subsequently, thecharge injected particles can be redispersed in another carrier that isuseful for applications, such as normal saline solution for medicalapplications.

Nanoparticles typically aggregate together, and this aggregation canlimit properties obtainable before and after subsequent processing. Onebenefit of charge injection, as well as the retention of this chargeinjection in substantially dried states, is a decrease in the degree ofthis aggregation and the ability to retain this decreased aggregationduring subsequent processing, such as in the formation ofnanoparticle/polymer composites (like carbon nanotube/polymercomposites).

Because of the free energy increase due to the contribution of highsurface area of nanoparticles and nanoparticles aggregates, thesecompositions can be used as high energy explosives and propellants.Methods of invention embodiments provide a way to appropriately adjust(and increase) the energy release capabilities of explosives andpropellants based on nanoparticles. The energy in taking anon-capacitively charged nanoparticle from the potential of the chargedstate (V) to the potential of zero charge (V_(pzc)) is ½C_(a)A(V−V_(pzc))², where C_(a) is the capacitance per unit area and Ais the effective surface area of the nanoparticle. This energy cancontribute to the energy released during explosions and propellantoperation. Also, the release of capacitively-stored energy can be usedto initiate explosion upon electronic contact of nanoparticlescontaining either opposite charge or differing amounts of charge of thesame sign. Such explosion can be initiated by mechanically contacting(and thereby electronically contacting) two or more solids comprisingparticles having differing signs or differing extents of chargeinjection. One application for such mechanical initiation of explosionis in safety air bags for vehicles. In order to maximize the gravimetricenergy release potential of the charged explosives and chargedpropellants of present invention embodiments, it is useful that thecounter ions used for charge injection have high free energy withrespect to reaction products produced by explosion and propellantburning. In addition, to this application of non-capacitive charging ofnanoparticles to increase the energy release during explosions or duringthe burning of a propellant, the additional heating effect associatedwith charging can be used to increase the effectiveness of chemicalwarming elements based on chemical reactions involving nanoparticles,and to regulate this warming process by control of charge release fromnanoparticles. Like the warming devices of the prior art, these warmingdevices can be used in shoes and other articles of clothing to increasethe comfort level of individuals exposed to freezing temperatures. Oneor more components of the above-mentioned charged energy releasematerials can usefully comprise electronically-conducting fiber, such asspun carbon nanotube fibers.

The process of charge injection changes the surface tension ofparticles, which can aid in dispersing particles in various materials,such as paint manufacture and generic polymer composite manufacture fromeither solution or melt phases. Also, this tunability in surface tensioncan aid in achieving appropriate dispersion of biologically-activecharged particles for medical applications. Examples are the delivery ofbiochemically active agents that are either the counter-ions to theinjected charge or the electronically charge injected particle, or acombination thereof. In addition to more direct biochemical effects, thebiochemical activity can result from heating the charge-injectedparticle using actinic radiation, such as infrared radiation ormicrowave radiation. Biochemically-active ions or ion-solvating speciescan either be delivered to the particles during the original chargeinjection process or during ion exchange or ion transformation processeslike those described in subsequent paragraphs. A sometimes achievablebenefit of using these latter processes is that valuablebiochemically-active agents can be accommodated with less waste of theseagents during processing. These species can be any of the variousbiochemically-active species that can serve as either anions, cations,or ion solvating species, like DNA, RNA, polypeptides (such as anenzyme, antibody, or aptamer).

After predominately non-faradaic charge injection (either for theparticles or a nanostructured solid), the charge-injected material canoptionally be contacted to a second material, called an “ionmodification material,” that either (1) provides ions that replace thefunction of the initial ions that are originally the counter charges tothe injected electronic charge, or (2) reacts with the initial ions toprovide ionic species, which then serve as counter ions to the injectedelectronic charge. The contacting material is here called an “ionexchange material” or an “ion transformation material,” depending uponwhether the result of contact with the charge-injected material is theion exchange process of (1) or the ion transformation process of (2).The benefit of such exposure can be to replace the original ions withions that provide either increased environmental stability or increasedresponse for the detection of agents in sensing applications.Alternatively, the result of this contact can be to provide ions thatcan be released as drugs or other chemicals during electronic dischargeof the charge injected material. The above mentioned contact can be byexposure of the charge-injected material to gaseous, liquid, or solidstates of either the ion exchange material or the ion transformationmaterial. One example of such processes is by immersing a hole-injectedmaterial containing BF₄ ⁻ into a salt solution containing the moreenvironmentally stable PF₆ ⁻. This exposure will result in at leastpartial replacement of the ambient unstable BF₄ ⁻ with the more stablePF₆ ⁻.

Another example, useful for medical applications, involves immersion ofthe charge-injected material into an aqueous solution containing DNA ora polypeptide, such as an enzyme, antibody, or aptamer. This exposurecan result in the exchange of the original ion with an ion of thebiological material and/or interaction of the original ion (such as H⁺)with the biological material to produce a new ion that comprises thechemical compositions of both the original ion and the biologicalmaterial. This biological material is now tightly bound to the chargeinjected material until decay of this charge injection causes release ofthe biological agent, which can be used for drug delivery. This decay ofcharge injected, and subsequent drug release, can result from exposureof the charge-injected material to redox active materials (such as thosepresent in the body), by contact between hole injected and electroninjected materials, the application of an electrical potential thatcauses current flow that decreases charge injection, and by heating thecharge injected material (such as by exposure of the charge injectedmaterial to infrared radiation in the infrared transparency region ofmammalian tissues or exposure to microwave radiation), or by exposure toother actinic radiation (such as γ-ray, x-ray, beta particle, or alphaparticle radiation). Most generally, the ion modification material cancomprise a biologically active component, or one that becomesbiologically active as a result of exposure to the charge-injectedmaterial. In some invention embodiments it is useful for thisbiologically active component to be a radioactive component, so thatdelivery of the charge-injected material to the mammalian body providesthe ability to deliver radiation to targeted tissues. Especially forthese biological applications, after the contact processes with the ionmodification material, excess ion modification material can be usefullyremoved from the charge-injected material (such as by drying). Prior toexposure to the mammalian body, the charge-injected material therebyobtained can be immersed in another agent that facilitates delivery ofthe biologically active material to the mammalian body. This agent istypically substantially biocompatible, like normal saline solution.While diverse materials can be used as the charge-injected material fordrug delivery, carbon nanotubes and shell-core metal particles areespecially useful for some application embodiments, since thesematerials provide infrared absorption in the transparency region of themammalian body. For the preparation of suitable shell-core metalparticles that can be non-faradaically charge injected using the presentart, see C. L. Loo et al., Technology in Cancer Research & Treatment 3,33-40 (2004) and references therein.

Semiconducting nanoparticles, especially those employed for color-basedbiochemical sensing, are useful for the practice of inventionembodiments. Examples include, but are not limited to, nanoparticles ofZnS, ZnSe, CdS, and CdSe. The particles for these invention embodimentsare typically less than 200 nm in their smallest dimension. Usingmethods that are above-described, such nanoparticles (or functionalizedderivatives thereof) can be conveniently charged electrochemically in anelectrolyte (either containing the desired counter ion, or a counter ionthat can be replaced by the desired counter ion during subsequentprocessing steps). This charging can be used to modify thedispersability of the charged, dried particles in liquids or melts,including polymer solutions or melts used to form nanoparticlecomposites. In one application mode, the semiconductor nanoparticles(charge injected using electrochemical processes of this invention) areused in composites that are employed for light emitting diodes andphotovoltaic cells. In another application mode, the charge-injectednano particles are used as color-based sensors, wherein the electroniccharge injection and associated counter ions effect the aggregation ofthese particles in response to contact with an analyte (which can bebiological). Such aggregation (or the interaction of the analyte withsingle charged nanoparticles) provides the spectroscopic response usedfor sensing, which is typically a color change or a change inluminescence.

The charge-injected materials of invention embodiments can be used asscaffolds for the growth of tissue in either culture media or inorganisms, including humans. Also, this charge injection can be used toincrease biocompatibility for devices implanted in the body. Theseinvention embodiments utilize Applicants' surprising discovery thatelectrochemically-injected charge is substantially retained when amaterial is removed from the electrolyte and optionally washed and ordried.

Biocompatibility of the surfaces of devices implanted in the body isimproved in some invention embodiments by overcoating these surfaceswith a porous material having a high gravimetric surface area, such asglassy carbon. Using the device with the over coated porous material asan electrode in an electrochemical cell, charge is injectedpredominantly non-faradaically in the porous coating. The type (holes orelectron) and degree of charge injection, as well as the nature of thecounter ion to the injected charge, can affect biocompatibility. Theinjected ions can be either those of the original electrolyte or thosethat substitute for the original ions during subsequent processes, suchas by immersion of the charge injected device into a solution containingthe replacement ions. As an alternative to this charge injection intoporous coatings that are preformed on the article to be implanted, suchas a pacemaker or an artificial heart, the electronic charge injectioncan be in a powder (such as nanofibers) that are first charge injectedand then used to overcoat the article. The sign of electronic chargeinjection is typically positive, although negative electronic chargeinjection can also be used (with some decrease in the lifetime oninjected charge in the body). The counter ions for the electronicallyinjected charge can include various inorganic, organic, andbiochemically derived species. Examples are Na⁺, Cl⁻, proteins(especially enzymes that are cellular growth factors), antibiotics, DNA,and RNA. Nanofibers (particularly nanofibers configured as porous sheetsand macrofibers) are particularly useful as substrate materials for thegrowth of tissue either in culture media or in animal or human bodies.Considerations on the choice of counter ions and the sign of chargeinjection are similar to those above-recited for surface coatings forimplanted devices.

Applicants' discovery that electrochemically injected charge is stablein the absence of electrolyte also enables the tuning of nanostructuredmaterials properties for materials absorption and desorption, such asfor hydrogen storage. The key point here is that charge injection (andassociated ion migration) selectively changes the interaction energy ofmaterials with a nanostructured substrate (such as nanostructuredfibers, sheets, and powders) and therefore changes the absorptioncapabilities of the substrate material. Discharge of this injectedcharge (such as by contacting oppositely charge-injected substratematerials) can aid in the release of absorbed materials.

Materials used for properties tuning by non-faradaic charge injection(including tuning for the above-mentioned chemical and drug releaseprocesses) generally have an achievable gravimetric capacitance forpredominately non-faradaic charging that is typically above about 0.1F/g, more typically above about 1 F/g, and most typically above about 10F/g. By achievable gravimetric capacitance, it is meant the capacitancemeasured when using an electrolyte and potential range that maximizesnon-faradaic charging and still yields predominately non-faradaiccharging. The gravimetric surface area of the materials used forproperties tuning by non-faradaic charge injection have an achievablegravimetric surface area (measured in nitrogen gas using the standardBrunauer-Emmett-Teller, BET, method) of typically above about 0.1 m²/g,more typically above about 5 m²/g, and most typically above about 50F/g.

Volatile electrolytes can be removed from the charged electrode materialmost simply by just evaporating the volatile component of theelectrolyte. Alternatively or additionally, the electrolyte can beremoved by washing with a second liquid that is typically not anelectrolyte for certain invention embodiments. For these inventionembodiments this second electrolyte is typically substantially free ofany salt. Ideally, this second liquid should be either miscible with theelectrolyte or capable of dissolving ions of said electrolyte. Thiswashing step or a sequence of washing steps using either the same ordifferent liquids can be usefully accomplished either before or after anoptional step in which the original electrolyte is wholly or partiallyremoved by volatilization.

Rather than removing the original electrolyte from the charge injectedelectrode, charge injection can be accomplished at a higher temperaturewhere this electrolyte serves as an effective electrolyte and then thisoriginal electrolyte can be cooled to a lower material use temperaturewhere the original electrolyte material has such low ionic conductivitythat it does not effectively serve as an electrolyte. This low ionicconductivity is preferably below 10⁻⁶ S/cm. This higher temperature istypically one at which the electrolyte is substantially liquid and thislower use temperature is typically one in which the electrolyte issubstantially solid. As an alternative to using temperature change toconvert a liquid electrolyte to a substantially non-conducting material,this transformation can be accomplished by polymerizing the liquidelectrolyte, for example, using thermal annealing, introduction of acatalyst, or exposure to actinic radiation (typically visible,ultraviolet, or higher energy radiation).

The liquid that is used to wash the electrolyte from the electrode istypically removed by volatilization. This liquid used for washing canoptionally and usefully be the solvent base of the electrolyte (such asthe acetonitrile of an original 0.1M tetrabutylammoniumhexafluorophosphate/acetonitrile electrolyte or the water in an original1 M aqueous NaCl electrolyte). However, in other useful embodiments ofthe present invention, the liquid that displaces the electrolyte ispolymerized while in contact with the electrode material. Thispolymerization can be accomplished, for example, using thermalannealing, introduction of a catalyst, or exposure to actinic radiation(typically visible, ultraviolet, or higher energy radiation).

Diverse properties can be tuned, and in many cases dramatically changed,as a consequence of the above process for electrochemically injectingcharge non-faradaically and retaining this charge in theelectrolyte-free state. Some of the effects by which charge injectioncan change material properties are: (1) the direct effect of changingband filling, (2) the effect of volume expansion and associateddimensional change due to charge injection, and (3) the effect ofcreating surface dipoles involving the injected electronic charge andcorrelated ions on the material surface. Material properties tuned bythe processes of invention embodiments include, for example, electricalconductivity, absorption and reflectivity (including color),thermopower, thermal conductivity, surface energy, and dielectricconstant. Materials with low conductivities in the non-charge-injectedstate (like forms of carbon, conducting polymers, and doped and undopedsemiconductors, exemplified by metal oxides and metal sulfides) can beused for invention embodiments that benefit from largecharge-injection-induced changes of conductivity, absorption andreflectivity (including electrode color), thermopower, thermalconductivity, surface energy, or dielectric constant.

Physical properties, such as electrical conductivity, opticalabsorption, magnetization, magnetoresistance, electromagnetic shieldingproperties, and the critical temperature of superconducting transition(T_(c)) depend very strongly on the number of charge carriers at theFermi level. Optional and useful materials of invention embodimentsinclude those that maximize the tunability of these properties that canbe achieved by predominately non-faradaic electrochemical chargeinjection. Such materials include those providing (1) a lowconcentration of charge carriers (electrons or holes) at the Fermi levelfor the uncharged state, and (2) a strong dependence of Fermi energy onthe amount of charge injection. This strong dependence of Fermi energyon charge injection is typically characteristic of the singularities indensity of states found for low dimensional conductors. Hence, 1-Dmaterials (like nanofibers and conjugated polymers like polythiophene)and 2-D layered materials (like cuprates of chalcogenides) are includedin some compositions. However, materials that can be intercalated (andthereby charge-injected faradaically) are optionally chargedpredominately non-faradaically. This can be done by either (1) choosingthe potential used for charge injection to avoid proximity to thepotentials where faradaic redox reactions occur or (2) by employinganions and cations that have unsuitable size for intercalation under thekinetic conditions used for charge injection. For example, bundledsingle-wall carbon nanotubes can be intercalated if the appliedpotential is either too high or too low. This intercalation can beavoided by suitably choosing the potential range so that intercalationdoes not occur or by using ions in the electrolyte that are too largefor facile intercalation (M. Stroll et al., Chem. Phys. Lett. 375,625-631 (2003)).

Materials used for electronic conductivity tuning by non-faradaicelectrochemical charge injection are optionally and preferablysemiconductors when the goal is to provide the maximum dynamic range oftunability. Materials having singularities in density of states near theFermi level can be used as materials having tunable electronicconductivity. These include the various well-known nanofibers, likesingle-wall and multi-wall carbon nanotube fibers.

The non-faradaically induced increase in electrical conductivity ofessentially electrolyte-free materials can be usefully employed formaking materials that combine high electrical conductivity with highoptical transparency. These transparent conducting electrodes are ofmajor importance for such applications as liquid crystal displays, lightemitting displays (both organic and inorganic), solar cells, switchabletransparency windows, solar cells, micro lasers, optical modulators, andoptical polarizers. Inorganic electrodes like ITO (indium tin oxide)degrade on bending and require costly vacuum based deposition methods.The embodiments of this part of the invention combines Applicants'discovery of processes for dramatically enhancing the electricalconductivity of nanostructured carbon nanotubes with prior-artdiscoveries related to the application of uncharged nanostructuredmaterials for making transparent conductors.

The prior art invention of A. Rinzler and Z. Chen (International patentnumber WO 2004/009884 A1 and PCT/US2003/022662 on “TransparentElectrodes from Single wall carbon nanotubes”) describes methods formaking electronically conducting, transparent SWNT-containing sheets.The described process enables the achievement of percolation betweenSWNTs in uniform coatings or films. The achieved sheet resistance is inthe range of 200 ohm/sq for an optical transmission of 30%, which can bedecreased to 50 ohm/sq for very thin films less than a hundrednanometers thick). However, the sheet resistance should be less than 10ohm/sq for applications in light emitting displays, solar cells, andother current dependent devices.

Presented here are invention embodiments in which the electricalconductivity of single-wall carbon nanotubes is increased by over anorder of magnitude, and retain much or all of this conductivityenhancement when the electrolyte is removed. An improvement thatApplicants bring is an increase in the level of electrical conductivitythat can be obtained, while still maintaining the desired degree ofoptical transparency. Carbon nanotubes that are non-faradaicallyinjected and largely electrolyte free are especially useful forinvention embodiments directed to highly conducting, opticallytransparent conductors. These conductors can be in the form of sheetcomposites or sheet coatings. Also, the carbon nanotubes can besingle-wall nanotubes, wherein the nanotubes can be either unbundled orhaving a small bundle diameter. These single-wall carbon nanotubes canbe hole injected, since Applicants have found that hole-injected carbonnanotubes provide much more stable electrical conductivity enhancementsthan do electron-injected carbon nanotubes. In order to insure longlifetimes for these non-faradaically hole-injected carbon nanotubes, itis generally important to use binder materials that do not containelectron donating impurities having redox potentials that enablereaction with the holes on the nanotubes. To insure that this is thecase, any binder composition should have sufficiently high ionizationpotential that electron transfer does not occur to the hole-injectednanotubes. Many conventional polymers, like polyethylene andpolypropylene, have this desired characteristic. Conventional methodscan be used to treat binder compositions so as to remove trace reactiveelectron donor impurities that might react with the holes in thehole-injected carbon nanotubes.

The work function of electrode materials generally is very important forapplications involving either hole or electron injection in organiclight emitting displays and for charge carrier collection for solarcells, as well as for related devices. The non-faradaic charge injectionof invention processes provides Fermi level and work function shiftsthat can be used to optimize this charge injection. The prior art workon multi-wall carbon nanotubes (M. Krüger et al., Applied PhysicsLetters 78, 1291 (2001)) has shown that the Fermi level increases by upto 0.3-0.5 eV upon electron injection, and decreases by up to 1 eV forhole injection, which will provide corresponding charges in the workfunction. These prior-art results are from electrochemical chargeinjection in liquid electrolyte. The enabling improvement provided bythe present invention embodiments is that it is shown that the typicallyrequired charge injection can be retained in the absence of contactingelectrolyte—either solid or liquid state.

The electron-injected electrolyte-free nanotubes (with decreased workfunction) can be used as electron-injecting electrodes for such devicesas OLEDs. On the other hand, the hole-injected electrolyte-freenanotubes (with increase work function) can be used as hole-injectingelectrodes. These hole-injected nanotube electrodes can be used toreplace ITO, since they also have large achievable work function and canbe used as an effective hole injector for OLEDs.

These device applications involving hole-injected and electron-injectedelectrolyte-free nanostructured materials as charge-injecting electrodestypically utilize device configurations in which an electrochemicalpotential is applied to the nanostructured material during deviceoperation. This is particularly important for the electron-injectednanostructured electrodes because charge can only be stabilized on theseelectron-injected electrodes in an inert environment, and oxygenexposure can cause degradation if the degree of electron injection ishigh. On the other hand, hole-injected nanostructured electrodes caneasily function without the need for either a continuously orintermittently applied electrochemical potential to refresh chargeinjection. Hence, these hole-injected nanotubes can be dispersed in asuitable unreactive binder and used as electrodes for devices, withoutany need for electrochemical charging after the initial chargeinjection. Transparent carbon nanotube fiber composites have alreadybeen used as replacements for low work function Al or Ag electrodes forplastic donor-acceptor solar cells (“Organic Photovoltaics” Eds. C.Brabec, V. Dyakonov, J. Parisi and N. S. Sariciftci, Springer Series inMaterials Science Vol. 60, 2003). These nanotubes were not substantiallycharge injected. The ability provided by the present inventionembodiments is to electrochemically tune the work function of thenanotubes for this application, without compromising desired performanceby the need of the prior art for either dopant intercalation or animbibed electrolyte.

Processes in accordance with some invention embodiments can be used forchanging the properties of largely electrolyte-free nanostructuredsuperconductors by substantially non-faradaic electrochemical chargeinjection. High temperature superconductors are especially useful, suchas members of the YBCO family. The most preferred compositions include,for example, LaSrCuO₂, YBa₂Cu₃O_(7−x), GdBa₂Cu₃O_(7−x),BiSr₂CaCu₂O_(8+x), and related cuprates. The compositional parameter xis typically in the range of 0.4 to 0.5 for YBa₂Cu₃O_(7−x). Furtherguidance for the superconductor compositions that are most suitable forelectrochemical non-faradaic charge injection of invention embodimentscan be found in C. L. Lin at al., Applied Physics Letters 71, 3284(1997). Although these authors use short-lived photo-injection of chargeto modify the superconducting properties, Applicants have found thatthose compositions that undergo the greatest changes in superconductingproperties upon photo-injection will also undergo the greatest changeswhen using the non-faradaic electrochemical charge invention of thepresent invention embodiments.

These superconducting oxides have low electron density at the Fermi, sonon-faradaically injected charge carriers will significantly changetheir properties. While the prior art has used charge injection tomodify the T_(c) of superconductors, this prior-art work has not usedpredominately non-faradaic electrochemical charging of a poroussuperconductor having a high gravimetric surface area. See, for example,X. Xi et al. in Applied Physics Letters 59, 3470 (1991), who use anon-electrochemical process for switching the superconducting transitiontemperature of films of YBa₂Cu₃O_(7−x) over a 2 K range. The achievedresistance modulation in the normal state can be as much as 20% and1500% near T_(c). Unlike the switching of superconducting properties inthe present invention embodiments, the T_(c) switching observed by Xi etal. is not practically useable for a macroscopic bulk superconductingmaterial and is unstable over the long term if connection to anelectrical power source is not maintained.

The non-faradaic charge injection of invention embodiment can be usedfor tuning superconductors for electronic transport and forelectromagnetic wave shielding and propagation for ultraviolet, visible,infrared, radio frequency, and microwave frequencies. A particularapplication embodiment is as superconducting elements for activefiltering, attenuation, phase shifting, and inter-line coupling formicrowave transmission lines (such as micro strip line, strip line andco-planar wave guides). Methods for using superconductors for microwavelines and modulated wave guides are well known in the art for themicrowave and radio frequency bands, and this technology can be used forapplication of the non-faradaically charge-injected superconductors ofthe present invention. Superconductors of the invention embodiments mayalso be used with photonic crystal arrays involving superconducting andsemiconducting elements to provide switching of photonic crystalproperties in the optional and preferred EM wavelength bands, such asmodulation of either the photonic band gap or the width and cut-offfrequency of metallicity gap. Other major applications for materialsmade by processes of invention embodiments are as superconductingtransmission lines, and magnets.

Perovskite manganites with the general formula R_(1−x)A_(x)MnO₃, where Ris a trivalent rare earth element and A is a divalent alkali earthelement, are included among the preferred compositions, where optionallyand most preferably R is La, Pr, Nd, or Sm, A is Ba, Ca, or Sr, and0.3<x<0.5. The reason for this preference is the sensitivity of theproperties of these materials to EBIG (Electrolyte-Bare Ion-Gated)charge injection, which makes them especially useful for EBIG materialsand devices. In fact, EBIG charge injection into these materials cancause transformation between insulating anti-ferromagnetic and metallicferromagnetic states.

The prior art has shown that the subtle balance between theanti-ferromagnetic insulating state and the ferromagnetic metallic statecan be shifted by application of external perturbations, like magneticfield (A. J. Millis, Nature 392, 147 (1998) and Y. Tomioka et. al.,Phys. Rev. B 53, R1689 (1996)); electric field (A. Asamitsu et al.,Nature 388, 50 (1997)); high pressure (Y. Morimoto et. al., Phys. Rev. B55, 7549 (1997)); exposure to X-rays (V. Kiryukhin, et. al., Nature 386,813 (1997)); or exposure to visible light (K. Miyano, et al., Phys. Rev.Lett. 78 (1997) 4257 and M. Fiebig, et al., Science 280 1925 (1998)).

In contrast with these prior-art approaches for the perovskitemanganites, Applicants either induce or sensitize transitions betweenthe insulating antiferromagnetic state and the conducting ferromagneticstate by using non-faradaic electrochemical charge injection that iseither tuned or maintained in the absence of contacting electrolyte. Thenon-faradaically electrochemically-induced phase transition of inventionembodiments can be used advantageously to replace the dielectric-basedelectric field effects of the prior art (S. Q. Liu et. al., Appl. Phys.Lett. 76 2749 (2000) and J. Sakai et. al., J. Appl. Phys. 90, 1410(2001)) in high density, nonvolatile memory devices. The colossalmagnetoresistance of the perovskite manganites can be tuned by the EBIG(electrolyte-bare ion-gated) methods of invention embodiments. Thistuning can be used for applications where a perovskite manganites actsas a device channel, which is switched by non-faradaic electrochemicalcharge injection from insulating and anti-ferromagnetic to metallic andferromagnetic. Such tuning is usefully employed for the spintronicdevices of invention embodiments, especially where a perovskitemanganite is between a magnetic source and drain. EBIG tuning changesthe sensitivity of the perovskite manganites to transitions induced byeither light or magnetic fields, and these effects can be usefullyemployed in devices. Additionally, EBIG materials can also be used forthe control of electromagnetic wave propagation for ultraviolet,visible, infrared, radio frequency, and microwave frequency regions,since modulation of electrical conductivity also changes the refractiveindex, dielectric constant, absorption, and optical reflectivity.

Devices of invention embodiments generally include at least threeelements: a working electrode, a counter electrode and a possiblymulti-component electrolyte material that helps provide an ionconducting path between the working and counter electrodes.Electrochemical charge injection in an electrolyte-free electrodecomponent is accomplished by applying a voltage between at least twoelectrodes (a working electrode and a counter electrode) that are bothin partial contact with an electrolyte, where an uninterrupted path forionic transport exists between these two electrodes. Charge injectioninto regions of the electrode that are not contacted with electrolyteoccurs, in most of the invention embodiments, by diffusion of the dopantions on the surface of a nanostructured material, such as a carbonnanofiber used as the channel of a field-effect transistor.

For invention embodiments where very high electrochemical charge anddischarge rates are desirable, the rate of surface migration of ions inan electrochemical electrode can be increased. Using at least twoelectronic contacts (typically at close to opposite ends of theelectrochemical electrode), a suitably high current is applied along theelectrochemical electrode in the direction where ion migration isdesirable. This current is referred to herein as the intra-electrodeion-migration enhancement current. Two processes produce enhanced ionmigration rate from this applied intra-electrode electronic current. Thefirst is electrode resistive heating, which increases ion mobility. Thesecond is the “electron wind force”. This electron wind force is wellknown to cause failure of small cross-section metal wires on circuitboards (through causing migration of atoms in the wire by the combinedeffects of electrostatic interaction with the electric field andmomentum transfer with electronic carriers due to scattering). Reversingthe direction of intra-electrode ion-migration enhancement currentreverses the direction of the electron wind force for a given ionicspecies (together with possible solvation sphere), so increasedmigration rates can be obtained for both electrode electrochemicalcharging and discharging. To achieve this beneficial effect of rateenhancement on both electrode charging and discharging, the direction ofthe intra-electrode ion-migration enhancement current can be reversed indirection when transitioning between electrochemical charge anddischarge processes. Use of these physical processes to enhance chargeand discharge rates for an electrochemical device is quite useful andhas not been previously shown in the prior art.

FIG. 9 schematically illustrates a means used in invention embodimentsfor injecting charge into a high-surface-area electrode that is onlypartially contacted with electrolyte. The charged state is pictured,where 900 is the electrode with positive injected electronic charge(electrostatically balanced by pictured anions that are in closeproximity, symbolized by the white spheres) and 901 is the counterelectrode with negative injected charge (electrostatically balanced withpictured cations, symbolized by black spheres). The nanostructuredelectrode materials for both 900 and 901 are single-wall nanotubes,although virtually any type of nanostructured conductor can be used (aslong as this conductor does not undergo degradative intercalation in thepotential range of device operation). As will be later described,material selection depends upon the device type and performance needs.Component 902 is an electrolyte, which only partially contactselectrodes 900 and 901. Element 903 is the variable voltage power sourceand associated leads that electrically connect to the two nanotubeelectrodes of 900 and 901. An applied potential from 903 injects chargeof opposite sign in the two pictured single-wall carbon nanotubeelectrodes 900 and 901. The required counter ions diffuse along thenanotube surfaces to enable this electronic charge injection. Reversingthe direction of current flow, by changing the applied potential, causesthe ions to diffuse from the nanotube surfaces back to the electrolyte902.

FIG. 9 pictures an arrangement where the ions used for electrodecharging are stored in the uncharged state in the electrolyte.Alternatively, ions can be shuttled between the electrode elements 900and 901 (or from and to optional additional electrodes) during thecharge and discharge processes. Additionally, a combination of theseelectrolyte storage and inter-electrode shuttle processes can beemployed. As will be later elaborated, one or more of the electrodes inan electrochemical device can be predominately faradaically charged anddischarged during device operation. Typically, a region of at least onedevice electrode is predominately non-faradaically charged anddischarged during normal device operation.

For comparison with the electrochemical semiconductor device of thepresent invention shown in FIG. 11, FIG. 10 illustrates a prior-artelectrochemical semiconductor transistor device. The illustratedprior-art device of FIG. 10 is referred to in the literature (M. Krüger,Applied Physics Letters 78, 1291-1293 (2001)) as being a “liquid-iongated” device. The device is separated from the substrate 1000 by aninsulating layer 1001 (typically SO₂). The device channel and anelectrochemical electrode (1002) is a semiconducting carbon nanotube,which is contacted by metal source and drain electrodes (1007 and 1008,respectively). The device is liquid-ion gated by using the variablepotential source 1006 and associated wiring to apply a potential betweenthe micropipette (1004) enclosed Pt wire electrode 1005 and the carbonnanotube electrode 1002, which is analogous to the channel of aconventional field effect transistor. The electrolyte 1003 covers boththe nanotube channel electrode and the Pt wire counter-electrode 1005.The device operates by using the power or signal source of 1006 toelectrochemically inject charge into the nanotube electrode/channel1002. This injected charge changes the electrical conductivity of thenanotube channel 1002, thereby varying the current that flows throughthe channel in response to a voltage difference applied between sourceelectrode 1007 and drain electrode 1008. This device is called aliquid-ion gated FET, since the nanotube channel 1002 is immersed in aliquid electrolyte and the ions in this electrolyte are needed for thecharge injection (i.e., gating) process. This or other prior art doesnot recognize that electrochemical double-layer charge injection canresult for electrode regions that are not in electrolyte, which is thereason for the illustrated complete immersion of the nanotube in theelectrolyte. This immersion of the channel 1002 in a liquid electrolyteis clearly problematic for ordinary transistor applications. The deviceof FIG. 10 could be used like a Chem-FET (chemical sensor based on aFET) for detecting materials dissolved in the electrolyte, by using theeffect of these materials on double-layer charge injection. However, ifthe material to be detected is a gas, this material must first dissolvein the electrolyte—which decreases device response rate and sensitivity.

FIG. 11 schematically illustrates an electrochemical transistor of thepresent invention embodiments that is not liquid-ion gated. This is anEBIG device since the electrolyte is not deposited to completely overlapthe device channel, and the region of the device that is bare ofelectrolyte facilitates device function. The device is built over atrench (1108) in an insulating substrate (1107). There are two devicechannels, and neither of these channels is in contact with a liquidelectrolyte. More generally important, the component of each channelthat largely determines gate resistivity is not surrounded by either aliquid electrolyte or a solid electrolyte—although at least an ioncomponent of this electrolyte must be contacting (together, optionally,with solvating species). There are two source and drain electrodes (1100and 1101, respectively) for the first leg of the device and two sourceand drain channels (1102 and 1103, respectively) for the second leg ofthe device. Likewise there are two semiconductor channels. The channelfor the first leg of the device (1104) and the channel for the secondleg of the device (1105) can be, for example, nanofibers (such as carbonSWNTs). A solid-state electrolyte (1106) lays over part of the sourceand drain electrodes and the channel for each leg of the device.

Device operation is as follows: Application of a voltage differencebetween electrode 1100 and 1102 (or gate electrode 1100 and 1103) causespredominately non-faradaic charge injection of opposite sign in channels1104 and 1105. This charge injection is enabled by the surface diffusionof cations (to electrostatically compensate for motion of electrons) forthe more negatively charged channel and by the surface diffusion ofanions (to electrostatically compensate hole motion) for the morepositively charged channel. This charge injection in the active channellengths (predominately the channel lengths that are suspended overtrench 1108) changes in a controllable way the electrical conductivityof the channels, which is indicated by a change in the current passingbetween electrodes 1100 and 1101 in response to an applied potentialbetween these source and drain electrodes (and between 1102 and 1103 inresponse to an applied potential between these electrodes).

The device of FIG. 11 (and a related device of FIG. 12) can be operatedas a replacement for a field-effect transistor, or in such applicationsas information storage or gas sensing. Because of the dual-leg nature ofthe FIG. 11 device, this device provides two transistor elements, twoinformation storage elements, and two sensors that are controlled in acorrelated manner.

When used as a chemical sensor for gas state species, a gas phasespecies interacts with the channel in a way dependent on the chargedstate of this channel, to thereby provide a change in the resistivity ofthis channel. Importantly, this and related devices of inventionembodiments can be used to do something that has heretofore beenimpossible—to do the equivalent of cyclic voltammetry for substancesthat are directly delivered to the device channel (or channels) in agaseous state. This unprecedented device capability results from thefact that materials in the environment of the channel will undergo redoxreaction with charges on the channel.

Like for ordinary liquid state cyclic voltammetry, the existence andrate of such reaction for a particular species in the gaseous state willdepend upon the redox potentials for these species. In standardliquid-state cyclic voltammetry, one scans electrode potential at aconstant rate and then plots the resulting inter-electrode current flowversus the applied potential. This process can be used for the presentcyclic voltammetry, with the important difference that the sensedmaterial is in the gas phase. For the device of FIG. 11 and devices ofrelated invention embodiments, additional sensor information can becollected that helps uniquely characterize gas phase species. Thisinformation includes the effect of such gas-state-delivered species onthe channel resistance as a function of channel potential, which istypically measured with respect to a reference electrode. Instead ofdetecting redox processes by using voltage scans and detecting theresulting current flow, a desired current can be caused to flow betweenelectrodes, and the resulting potential of working and referenceelectrodes can be monitored versus time. Additionally, resistive oroptical heating of the channel can be used to cause desorption ofmaterials from the channel, and thereby provide another means forcharacterizing an analyte.

The device pictured in FIG. 11 does not provide a reference electrode,which is typically used in liquid state cyclic voltammetry. Such areference electrode, or more than one reference electrode, can beusefully incorporated in the device of FIG. 11 by including a referenceelectrode material (such as a platinum wire or a platinum film) inelectrolyte 1106. This reference electrode (or a multiplicity of suchreference electrodes) should not be in electrical contact with otherelectronically conductive elements of the device. Measurement of thepotential of a channel with respect to a reference electrode (which canbe located in close proximity) enables placement of channel potentialson an absolute scale, so that the redox potentials of detected gas phasespecies can be most reliably determined. The two channel materials inFIG. 12 need not be identical or even comprise a nanofiber.Specifically, the use of film strips for one or more of these channelsis also included in some invention embodiments. However, these channelmaterials should typically have both semiconducting and highlyconducting states and the possibility of transitioning between thesestates as a result of charge injection. One of these channel materialscan optionally also be a material that is predominately chargedfaradaically in the potential range of device operation, such as aconducting organic polymer or vanadium pentoxide nanoribbons (see G. Guet al., Nature Materials 2, 316-319 (2003) and provided references fordescription of the synthesis, properties, and self-assembly of thesevanadium pentoxide nanoribbons).

FIG. 12 schematically illustrates a second electrochemical transistordevice of the present invention that does not use liquid-ion gating, andwhich could be used for information storage or gas sensing. Like thedevice of FIG. 11, this is also a EBIG device—but unlike the device ofFIG. 11 the present device has only one channel. The device isconfigured over a trench (1207) in an insulating substrate (1206). Theoperation and benefits of this device are similar to that of the devicein FIG. 11. A potential applied between gate electrode 1202 and sourceelectrode 1201 controls the amount of charge injection in the channel1203, which can be a semiconductor when there is no charge injection.This charge injection charges the electrical conductivity of thechannel, which is measured by applying a voltage between sourceelectrode 1201 and drain electrode 1200 and measuring the resultingcurrent flow through channel 1203. A material 1204 capable of chargeinjection overlies the gate electrode and is electronically part of thisgate electrode. This material can undergo charge injection eitherpredominately non-faradaically or predominately non-faradaically in thegate-source voltage operation range of the device. The solid-stateelectrolyte 1208 contacts the material 1204 and the channel material1203, and provides an ion-conducting path between these elements.However, the measured conductivity of the channel is in large partdetermined by regions of the channel that do not contact theelectrolyte, and the charging of this channel can be predominatelynon-faradaic in the typically utilized operation range of the device.

Well-known prior-art methods can be used for the fabrication of thedevices of FIGS. 11 and 12, and these methods will be later provided ina general discussion of methods generically useful for the fabricationof devices from material elements that can have nanoscale dimensions.Examples of the many dozens of papers in the literature that teach oneskilled in the art to fabricate individual nanofiber devices are J. A.Misewich et al., Science 300, 783-786 (2003), L. Gangloff et al., NanoLetters 4, 1575-1579 (2004), and G. S. Duesberg et al., Nano Letters 3,257-259 (2003).

Devices like that shown in FIG. 12 can also be used a chemical sensorsof materials that are delivered in liquid states. A variety of processescan be used for the sensing. One process is ion exchange between thechannel material and ions of the analyte. Another process is reaction ofions of the analyte with ions originally present as counter ions to theelectronically injected charge. A third process is redox reaction ofinjected charge with the analyte. Selectivity of response can beachieved for sensor applications by incorporating biochemicallyselective species in the device channel material, such as DNA, RNA, andpolypeptides (including, for example, antibodies, enzymes, andaptamers). This incorporation can optionally be accomplished for thedevice channel by exposure of the device channel in the non-faradaicallycharge injected state to large biological molecules, like DNA, RNA, andpolypeptides. Such exposure can result in either the replacement of theoriginal ions with such large molecules in the charged state or reactionof the original ions with these large molecules to produce new iontypes.

These biological molecules (like DNA, RNA, and polypeptides) arenormally charged, so they are suitable counter-ions to electronicmaterials injected in an electrode material. Also, they can providerecognition functions that are very useful for sensing. However, becauseof large molecular size they do not have high mobility on surfaces.Hence, these materials are most preferably delivered to a surface (suchas a device channel) from a liquid media. This delivery (such as fromnormal saline solution) can be optionally accomplished during devicefabrication, and this liquid and unneeded salt can be optionally removedlater during device fabrication by evaporation and washing processes.Because of the low mobility of these large biological molecules onsurfaces, switching of the degree of ionic charge on these moleculesduring device operation is typically accomplished by using electrolytesthat provide ionic species having high mobility, like the H⁺ ion inNafion. In such case, the electronically controllable degree ofincorporation of the H⁺ ion in the biomolecules determines the amount ofionic charge on the biomolecules, which in part determines thesensitivity of a channel comprising such a biomolecules to analytes,such as a structurally matched strands of DNA. Hybridization of a DNAstrand on a device channel with an analyte DNA strand is an especiallyuseful sensing mode.

Is well known in the prior art that faradaic charge injection canprofoundly affect the optical properties of material and that thesefaradaically-changed optical properties can be maintained in the absenceof an electrolyte. However, prior-art investigators have not discoveredthat optical properties changes can result from non-faradaic chargeinjection that is accomplished without direct contact of an ionicallyconducting material (such as an electrolyte or electronicallyintercalated conducting polymer).

The optical device schematically illustrated in FIG. 13, which isillustrative of many related devices of the invention embodiments,utilizes the above-described discoveries of Applicants'. Devices of thisand related types of the invention embodiments provide the switching oroptical properties of a material region that is not directly contactedwith an electrolyte or an intercalated material that provides iontransport. Typical applications of these devices are gas sensors basedon surface enhanced Raman (SERS) or fluorescence, infrared camouflagelayers, and electronically switchable photonic crystal mirrors.

FIG. 13 schematically illustrates an optical gas sensor, based on thesurface-enhanced Raman effect, that uses electrochemically controlledcharge injection in a metallo-dielectric photonic crystal to optimizesensitivity and species selectivity. The desired device performanceenhancement originates from a number of possible effects, including (1)the charge-injection-tuned pickup of gas phase components that are to besensed, (2) the optimization of the resonant effect of SERS bycharge-injection-based tuning of plasma frequency (and therefore theresonance enhancement of the SERS effect), and (3) concentration of thegas-phase components on the high surface area of the electrode used forsensing.

Element 1300 of FIG. 13 is an inverse-lattice photonic crystal, whichalso functions as a working electrochemical electrode. This element ispreferably a conducting photonic crystal having a void volume of greaterthan about 50%. This element is optionally an inverse-lattice photoniccrystal that is comprised of a high reflectivity metal, like silver(which is especially suitable). Element. 1302 is a solid-stateelectrolyte that contacts the photonic crystal. The solid-stateelectrolyte preferably has low electronic conductivity and an ionicconductivity of above 10⁻⁴ S/cm at room temperature. Element 1301 is acounter electrode to the working electrode 1300, which is also aphotonic crystal. This counter electrode can be one that operatespredominately faradaically or predominately non-faradaically.Intercalated conducting polymers that undergo predominately faradaiccharging during device operation or very high surface areanon-intercalated materials (like nanoporous Pt or nanofibers) areespecially suitable for use as this counter-electrode. This counterelectrode element 1301 can optionally be a second photonic crystal thatcan be predominately charged non-faradaically during device operation.The benefit of using two photonic crystals is that one obtains twomaterials for SERS sensing (one with negatively injected charge and theother with positively injected charge), which can be simultaneouslyprobed optically during device operation. Element 1303 is a variablevoltage or variable current power supply, and associated electricalwires to the working electrode (1304) and the counter electrode (1301).Item 1304 in FIG. 13 indicates the input and output of light to thephotonic crystal, which need not be in the pictured orthogonal directionto the photonic crystal surface. The electrolyte element 1302 typicallyincludes a reference electrode (not shown) for placing measuredpotentials on an electrochemical scale—so that the degree of chargingcan be determined from the measured potential of 1300 with respect tothis reference electrode, and used to control the degree of chargeinjection for 1300.

Various methods well known in the art could be used to make the deviceof FIG. 13 and related devices, as either devices that are macroscopicor macroscopic in lateral area, and optionally probed either above thediffraction limit of light or using well known photon tunneling methods(that avoid the diffraction limitation of light). Methods forfabricating a metallo-dielectric photonic crystal 1300 (which can befrom a high reflectivity metal like silver) are described, for example,by A. A. Zakhidov et al., U.S. Pat. No. 6,261,469 and U.S. Pat. No.6,517,762, L. Xu et al., Advanced Materials 15, 1562-1564 (2003), L. Xuet al., J. Am. Chem. Soc. 123, 763 (2001), and O. D. Velev et al.,Nature 401, 548 (1999). Fabrication of the device can proceed, forinstance, by using methods of these references (and optionally usingconventional lithographic methods) to make photonic crystals 1300 orarrays of photonic crystals (if an array of many devices are required)on a substrate. Subsequent deposition of electrolyte 1302 and counterelectrode 1301 (for instance, a conducting organic polymer) can, forinstance, be accomplished by spin coating 1301 and 1302 (again withoptional usage of conventional lithographic methods to define devicesize) on 1300. The thereby fabricated devices can be transferred to asecond substrate using conventional wafer bonding methods, followed byeither chemical or mechanical removal of the first mentioned substrate,so that each photonic crystal is obtained as the top layer on the secondsubstrate. The electrical connections to the photonic crystal 1300 andthe conducting polymer in each device can then be lithographicallydefined. More simply, employing a slab of photonic crystal as thesubstrate and depositing the electrolyte 1302 and counter electrode 1301on one side of this substrate can eliminate the need for these first andsecond substrates.

Application of the device of FIG. 13 as a SERS based gas sensor is asfollows: Applying a potential between the working electrode 1300 and thecounter electrode 1301 injects charge into the photonic crystalelectrode (1300), thereby modifying the adsorption of targeted materialsonto the external and internal surfaces of 1300 and appropriatelyshifting the frequency of surface plasmons. Different gas components canbe selectively detected by measuring the SERS spectra during scanningthe degree of charge injection in 1300. Either this device scanning isat a sufficiently slow rate that surface adsorption and desorption canbe accomplished at ambient temperature or heating of component 1300 canbe used to accelerate these surface absorption and desorption processes.This heating can be accomplished either electrically by resistanceheating or by the heating effect of radiation adsorption. Monitoring theSERS spectra during heating and cooling processes is usefully employedfor obtaining additional information about the composition of the sensedgas. Additionally, measurement of current flow between the working andcounter electrode as a function of the potential of these electrodes (orthe electrode potential versus applied inter-electrode current and time)can be used in combination with the SERS spectra to characterizeanalytes.

Like for most of the devices of invention embodiments, the device ofFIG. 13 can be operated either in a “rocking chair device” mode or in an“electrolyte ion storage” mode, or as a combination thereof. In therocking chair mode, ions are shuttled between working and counterelectrodes during device operation, and the function of the electrolyte(1302) is just to electronically insulate the working and counterelectrodes and to enable ionic transport between them. In theelectrolyte ion storage mode, the electrolyte stores the ions that areinjected into opposite electrodes during device operation (typicallyanions for one electrode and cations for the opposing electrode for agiven change in inter-electrode operation voltage). If there is only onemobile ion in the device system, then the device operation will be bythe rocking chair mode, which requires only sufficient electrolyte 1302to insure that the working electrode (1300) and the counter electrode(1301) are electronically insulated with respect to each other and thatthese electrodes are intimately contacted by the electrolyte. Althoughdevice operation in either of these modes can be usefully employed forthis and other devices of invention embodiments, device operation inpredominately the electrolyte storage mode can be advantageous. Thereason is that operation in strictly a rocking chair device moderequires that at least one of these electrodes is charge injected duringdevice fabrication. Operation in the electrolyte ion storage moderequires electrolytes in which both anions and cations are mobile. Thiscontrasts with the case of fuel cell devices of invention embodiments(based on either hydrogen or hydrocarbon fuels), where electrolytesproviding predominately H⁺ ionic conduction are optional and morepreferred, because of the need for H⁺ ion conduction for these fuelcells and the relatively high electronic conductivities of many H⁺transporting electrolytes.

Devices having the basic configuration shown in FIG. 13 can also be usedto switch other properties of the working electrode of this figure,especially magnetic properties, electrical conductivity, microwaveabsorption, surface energy, thermal diffusivity and thermalconductivity, thermopower, the existence and characteristics ofsuperconductivity, and surface friction coefficients. For all of theseapplication modes except switching surface friction and switchingsurface energy, it is advantageous that the working electrode materialcomprises at least 20% void volume. More advantageously, the void volumeis at least about 50%, and most advantageously, this void volume is atleast about 75% for the working electrode. Specific compositionssuitable for these types of devices are listed elsewhere in thissection.

The presently discovered ability to electrochemically inject charge intoelectrode regions that are not in direct contact with electrolyte isespecially useful for fuel cell devices. According to conventionalthinking, fuel cell redox reactions (oxidation or reduction of fuel cellreactants) occur at locations where the catalyst, fuel cell reactant,electrolyte, and electrode come in joint contact. The device embodimentsin the fuel cell area utilize Applicants' discovery that redox reactions(either faradaic or non-faradaic) can occur for electrode regions thatare not in contact with electrolyte. One consequence of this discoveryare devices providing greater catalyst utilization, since efforts to atleast partially contact the electrode with electrolyte results in manycatalyst particles being buried in electrolyte, thereby making thesecatalyst particles inactive—since they are not exposed to the fuel cellreactant (which is often a gas). The result of ineffective use ofcatalyst is high catalyst cost for the fuel cell. Another consequence ofthe discoveries used in invention embodiments is high achievablegravimetric current densities, since a high gravimetric surface areaelectrode is used and most of the catalyst on this surface area isactive.

FIG. 14 schematically illustrates a fuel cell in accordance withinvention embodiments, which is enclosed by element 1400. Elements 1401and 1402 are carbon nanotube forests along with associated H⁺ cations(1407) and O⁻² anions (1408) resulting from redox reaction of the fuelcomponents (H₂ and O₂). By carbon nanotubes forests it is meant an arrayof vertically aligned nanotubes. These forests contain catalystparticles 1406, such as Pt nanoparticles. Elements 1403 and 1404 areelectronically conducting sheets that are H⁺ ion conductors (such aspartially protonated, conducting polyanaline) that are electricallycontacted on the left to withdraw the electrical energy produced by thefuel cell. These sheets (1403 and 1404), which make electrical contactto the nanotubes, are separated by a sheet (1405) of a proton-conductingelectrolyte of the type used for conventional fuel cells, such ashydrated Nafion. The large spheres 1409 symbolize the negatively chargedpolymer in this electrolyte. This fuel cell works by the oxidation of H₂to produce protons (H₂→2H⁺+2 electrons) on the top nanotube forest 1401and by the reduction of O₂ to produce O⁻² (O₂+4 electrons→2O⁻²) on thebottom nanotube forest (1402). Unlike in the case of prior art fuelcells, these redox reactions predominately occur in the gas phase (onsurfaces of the nanotube forest that are exposed to the H₂ and O₂ gas).H⁺ ions transverse the electronically and ionically conducting sheet1403, the electrolyte sheet 1405 (ionically conducting), and theelectronically and ionically conducting sheet 1404 to react with the O⁻²ions to produce an overall cell reaction (O₂+2H₂→2H₂O). The reduction atelectrode 1401 and the reduction at electrode 1402 provides a cellpotential of approximately a volt that produces a current throughelectrically powered devices that are attached to the indicated + and −electrodes (shown on the left hand side of the figure).

Various methods well known in the art can be used to fabricate the fuelcell device of FIG. 14. For example, the synthesis of either singlewalled or multiwalled nanotube forests (like the pictured nanotubearrays) by chemical vapor deposition (CVD) and by plasma-enhanced CVD isdescribed in the literature by various authors (see S. Fan et al.,Science 283, 512-(1999), A. M. Casell et al., Langmuir 17, 260-(2001),and L. Delzeit et al., J. Appl. Lett. 91, 6027-(2002)). A conductingpolymer (like conducting polyaniline) can be chemically polymerized,deposited from solution, or electrochemically polymerized on top of thenanotube array that is on the growth substrate. Then the conductingpolymer with attached nanotube forest can be stripped from the growthsubstrate to free the combined elements 1401 and 1403. The nanotubeforest 1402 and contacting conducting polymer 1404 can be analogouslyfabricated. Both of these nanotube forests and supporting conductingpolymer layers can be connected on opposite sides of a freestandingpolymer electrolyte membrane 1405 to thereby complete the complicatedaspects of device fabrication. This connection can be made, for example,by using a solvent or swelling agent to soften either the electrolytelayer or the conducting polymer layers 1403 and 1404 (or both), solamination of the electrolyte and conducting polymer layers isfacilitated. The nanotube forest electrode 1401 can be optionallyreplaced with various other nanostructured materials, such as the porouscarbon networks loaded with catalyst. See A. A. Zakhidov et al. inScience 282, 897 (1998), U.S. Pat. No. 6,261,469, and U.S. Pat. No.6,517,762 and J.-S. Yu et al., J. Am. Chem. Society 124, 9382-9383(2002) for details on how these porous carbon networks can besynthesized by opal templating, and filled with catalyst particles.

While the fuel cell reactants shown for the device are H₂ and O₂ (whichcan be oxygen in air), other fuel cell fuel couples (in either gaseousor liquid states or a combination thereof) can be more generically usedfor invention embodiments. These include, for example, methanol andhydrazine and the oxidant hydrogen peroxide. The fuel component that isoxidized is optionally and most preferably in gaseous form and isoptionally and most preferably either hydrogen; a hydrocarbon such asCH₄, C₂H₆, or C₃H₈ at preferably 100-200° C.; an alcohol such asmethanol or C₂H₄(OH)₂ at preferably 20-80° C.; H₂S at preferably 20-90°C.; a nitrogen derivative such as NH₂NH₂ at preferably 20-60° C.; orammonia at preferably 200-400° C. The fuel component that is reduced isoptionally and most preferably oxygen.

Examples of proton-conducting electrolytes that are useful for the fuelcell device of FIG. 14 (and in related devices) are Nafion, S-PEEK-1.6(a sulfonated polyether ether ketone), S-PBI (a sulfonatedpolybenzimidazole), and phosphoric acid complexes of nylon, polyvinylalcohol, polyacryamide, and polybenzimidazole(poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole]. These and other usefulproton conductors for use at either ambient or higher temperatures aredescribed in G. Alberti et al., J. Membrane Science 185, 73-81 (2001);G. Alberti and M. Casciola, Solid Sate Ionics 145, 3-16 (2001); P. L.Antonucci et al., Solid State Ionics 125, 431-437 (1999); L. Jörissan etal., J. Power Sources 105, 267-273 (2002); A. Bozkurt et al., Solid SateIonics 125, 225-233 (1999); and T. Norby, Solid Sate Ionics 125, 1-11(1999). As an alternative to Pt, various other fuel cell catalysts canbe used, including Ru and such Pt alloys as Pt—WO₃ and Pt—Ru and othercatalysts given in B. Rajesh et al., Fuel 81, 2177-2190 (2002). Thiscatalyst can be added to the nanotube arrays either after initialsynthesis of the nanotube forest, or after subsequent process steps byusing well-known methods (see, for example, B. Rajesh et al., Fuel 81,2177-2190 (2002); C. Wang et al., Nano Letters 4, 345-348 (2004); W. Liet al., J. Phys. Chem. B 107, 6292-6299 (2003); X. Sun et al., Chem.Phys. Lett. 379, 99-104 (2003); G. Che et al., Nature 393, 346-349(1998); and J.-H. Han, Diamond and Related Materials 12, 878-883(2003)).

While carbon nanotube forests are used here to illustrate the devicemethodology, any of a number of nanoporous conductors can provide thefunction of elements 1401 and 1402, such a sheets of nanofiber paper(such as carbon nanotube paper) and membranes made by the growth ofcarbon nanotubules in the pores of alumina membranes (see G. Che et al.,Nature 393, 346-349 (1998) and B. Rajesh et al., Fuel 81, 2177-2190(2002)). The electrode elements 1401 and 1402 need not be made of thesame material. In fact, one of these electrode elements can be a fuelcell element of the prior art that is essentially fully infiltrated withelectrolyte.

Devices based on electrochemically tunable charge injection on theinside of hollow nanofibers are also fall within the scope of thepresent invention embodiments. Such devices are especially useful forapplications where the presence of ions on nanofibers is problematic.For example, it is sometimes useful to tune the work function (andtherefore the electron emission properties) of nanotube fibers by chargeinjection for applications where these nanotubes are used for the fieldemission of electrons. These nanotube field emitters operate in vacuumand the presence of mobile ions on the external surface of nanotubes ornanotube bundles can cause undesirable fluctuations in electron emissioncharacteristics, and stability in electron emission is criticallyimportant for such applications as cold cathode emitters for highresolution electron microscopes. In these invention embodiments, theinside of the hollow nanotube fibers can be contacted with all of thecomponents of an electrolyte. However, in states accessed during normaldevice operation, the material inside the nanotube will differ fromconventional electrolytes in that these is a net excess or either anionsor cations.

FIG. 15 schematically illustrates a tunable nanotube device in whichtunability results from electrochemically induced insertion of ionsinside a nanotube, and insertion of associated counter electroniccharges onto the nanotube. One particularly important tunable propertyis electronic work function. An important type of device having the FIG.15 configuration is a electron field emitter that can be used, forexample, as an electron emitter for field-emission displays, electronmicroscopes, and x-ray sources. Increased stability of electron emissionintensity is one advantage of this particular invention embodiment, ascompared with other invention embodiments of this invention where ionsare electrochemically placed on either the inside of the nanotube or onboth the inside and outside of the nanotubes. The case is especiallyuseful when the ions electrochemically inserted into the interior ofsaid nanotube are at least 10 times more numerous than thoseelectrochemically inserted on the exterior surface of said nanotube

The device shown in FIG. 15 contains electrochemically-active electronicconductors 1500, 1502, and 1506. Elements 1500 and 1502 can optionallybe materials that can be intercalated, so they electrochemically chargepredominately faradaically. Element 1500 is the counter electrode to aworking electrode that comprises the electronically interconnectedelements 1502 and 1506. Element 1501 is an electrolyte that separatesthese working and counter electrodes. Element 1506 is an electronicallyconducting nanotube that is closed at one end and open at the oppositeend. Element 1504 (and like elements in electrolyte 1505 and inside thenanotube element 1506 are mobile cations. Elements 1505 in theelectrolyte 1501 are anions, which can be either mobile or immobile. Thetotal charge of on these anions matches the total charge of the cationsin the electrolyte 1501. In contrast, the total ion charge in elements1500, 1502, and 1506 is matched in large part by electronically injectedcharge in these elements. Element 1503 is an ion permeation barriermaterial, such as a deposited metal, which restricts ion placementduring charging to filling of the nanotube 1506. This element 1503 canoptionally be omitted. One disadvantage of this omission is that theresulting partial placement of ions on the exterior surface of thenanotube can decrease the stability of electron emission. An advantageof omitting element 1503 is that higher amounts of charge can beinjected into element 1506 (at a given applied potential between 1500and 1502) than would be the case if ion placement during charging wererestricted to only the inside of the nanotube element 1506. Element 1506is a variable potential power source and associated electronicconnections that enables the application of a potential between thecounter electrode 1500 and the working electrode that comprises 1502 and1506. Element 1508 is a power source that applies a potential betweennanotube 1506 and a counter electrode (1509) to thereby cause electronemission.

The device of FIG. 15 operates as follows. Ions shuttle between thecounter electrode (1500) and the working electrode (comprising 1502 and1506) depending upon the potential applied between these electrodes by1507. The degree of charge injection varies in a controllable mannerwith the work function of nanotube element 1506. Electron emissionoccurs (predominately from near the tip of nanotube element 1506) inresponse to the application of an applied voltage between 1506 and 1509.

The choice of materials for the device of FIG. 15 can be made in varioususeful ways depending upon performance considerations for a particularapplication. Also, variations on this device design are useful. Forexample, there is an excess of cations in the device of FIG. 15, andthis excess of cations is stored in the working electrode (comprising1502 and 1506) and counter-electrode (1500). The counter charges areinjected electrons in these electrodes. In normal operation, thispictured device operates as an above mentioned rocking chair mode, whereions are shuttled between working and counter electrodes during deviceoperation, and the function of the electrolyte (1505) is just toelectronically insulate the working and counter electrodes and to enableionic transport between them. While the ions shuttled in FIG. 15 arecations, other invention embodiments involve the inter-electrodeshuttling of cations. Also, devices of invention embodiments can beconstructed analogously so that operation is in the electrolyte ionstorage mode. The only basic difference between the device of FIG. 15and a related device of invention embodiments that operates in theelectrolyte storage mode is that both anions and cations used for deviceoperation predominately come from the electrolyte, so these ions mustboth be mobile. The electrode elements 1500 and 1502 can be either ananoporous material that charges predominately non-faradaically or anelectrode material that charges predominately faradaically, such as lowsurface area conducting polymer. The nanotube element 1506 canoptionally be a multiwall nanotube, instead of the pictured singlewalled nanotubes. However, this multiwall nanotube should typically beclosed on one end for at least one of the nanotubes in the multiwallassembly (in order to enhance the stability of electron emission). Thenanotube element 1506 can either have an open end that is located inelement 1502 or an open end that opens into the electrolyte 1505 (aspictured in FIG. 15), as long as this nanotube makes electronic contactwith element 1502. These nanotubes can be carbon nanotubes or any of theenormous variety of conducting materials that are known to form eithersingle walled nanotubes or multiwall nanotubes (including double wallednanotubes). Also nanotube element 1506 can comprise a bundle of singlewalled nanotubes, a bundle or multiwall nanotubes, or a mixture thereof.The aspect ratio of the nanotube or nanotube bundle element (ratio ofdiameter to exposed length) is important for determining the fieldenhancement factor, as is the absence of proximity similarly highneighboring conductors. Specific compositions useful for the practice ofinvention embodiments are described later, when the various possiblecompositions and comparative advantages and disadvantages of thesecompositions for various application embodiments are described.

Devices of FIG. 15, and related devices of invention embodiments,contrast with those of the prior art. Consider the application of theFIG. 15 device for field emission of electrons. Carbon nanotubes arecurrently widely studied as a source for electric-field-induced electronemission (so-called cold cathode electron emission). They are alreadyused as a cold cathode for numerous applications, such as Field EmissionDisplays (FEDs) and x-ray sources. Invention embodiments enablegate-controlled electrochemical charge injection in carbon nanotubes forthe optimization and control of the electronic work function of thecarbon nanotubes, and therefore their emission characteristics. Whenelectrons are injected into the nanotubes the work function decreases,and when holes are injected the work function increases. Inventionembodiments provide electronically tunable cold cathodes for flat paneldisplays, lighting fixtures, electron sources for electron microscope,and discharge tubes for over-voltage protection.

The next described devices of invention embodiments are supercapacitorshaving greatly reduced self-discharge and the ability to be dry-shipped.Prior art supercapacitors are based on continuously maintained contactof electrolyte with the capacitor electrodes. This electrolyte contactcauses two major problems. The first problem is that the presence ofthis electrolyte means that charge cannot be stored in supercapacitorsover long time periods. This self-discharge cannot be eliminated as longas electrolyte interconnects the two electrodes, since (a)self-discharge results from the small electronic component ofelectrolyte conductivity and (b) redox mediators can exist in theelectrolyte, which undergo oxidation at one electrode and reduction atthe opposite electrode—to thereby shuttle electronic charge betweenelectrodes. If the first problem could be solved then it might bepossible to ship charged supercapacitors for later use in the field. Asecond problem then arises, since the continuously maintainedelectrolyte in the supercapacitor increases the supercapacitor shippingweight. The experimental discoveries involving this invention show thatinjected charge in electrodes can be maintained even when theelectrolyte is absent, and that self-discharge is inherently lower thatfor the case where the supercapacitor electrodes are immersed inelectrolyte.

FIG. 16 schematically illustrates a supercapacitor of inventionembodiments that can be charged, drained of electrolyte, partially orcompletely evacuated then reactivated for subsequent discharge in aremote location by refilling with electrolyte. In the illustrated case(showing a cross-section of the device normal to the supercapacitorelectrode sheets (1601 and 1602)), the electrolyte for device refill iscarried in a compartment of the device. In an alternative inventionembodiment, the electrolyte (which can be salt water) is injected intothe supercapacitor device from an electrolyte container that is separatefrom the device. A benefit of this alternative design is that thesupercapacitor can be dry shipped in charged state without the need tosimultaneously transport the electrolyte.

While batteries can provide much higher energy storage densities thansupercapacitors, the power densities of supercapacitors can be muchhigher than for batteries, which is what makes this invention embodimentimportant. Also, Applicants have discovered that injected charge is muchmore stable in the absence of contacting electrolyte than it is for theelectrolyte-containing supercapacitor. In the device of FIG. 16 (incontainer 1606), non-faradaically injected charge is stored in thehole-injected electrode (1601) and in the electron-injected electrode(1602) in the absence of contacting electrolyte. Subsequent release ofthis stored energy is enabled when electrolyte (1608) in the device ispushed into the supercapacitor compartment that contains the sheetelectrodes 1601 and 1602 and the porous insulator sheet (1603), whichprevents short circuiting of these electrodes. Application of a force tothe moveable element 1607 breaks a membrane (not shown) separating thesupercapacitor electrodes and separator sheet from the electrolyte andpushes the electrolyte into the active cell volume, as shown in 1609(which is the supercapacitor device after release of the electrolyteinto the region of the electrochemically active cell elements). Beforethis electrolyte release, the electrical contacts (1604 and 1605) to theelectrochemical electrodes are electrically floating, but after therelease of electrolyte these electrical contacts acquire the relativepotentials shown in 1609.

The invention embodiments also include a supercapacitor/battery hybriddevice in which at least one (and in some cases both) of the deviceanode and cathode comprise elements that that are predominatelyfaradaically charged and predominately non-faradaically charged. Thecomponent that is predominately non-faradaically charged is typicallyexterior to the component that is predominately faradaically charged,such as being a covering a sheet on one or both sides of a sheet of thepredominately faradaically charged material. These predominatelyfaradaically charged electrode components should be in electricalcontact and advantageously in close physical contact. Thissupercapacitor/battery hybrid device has the advantages of devices likethat shown in FIG. 16 in that the device can be stored and shipped in adry state without contacting electrolyte, and then electrolyte can beplaced in contact with the electrodes immediately prior to usage,thereby increasing charge storage life over the life that would resultif the device electrodes were in maintained joint contact with theelectrolyte. In this “dry state” of the supercapacitor/battery hybridenergy storage device there is no complete ion path in an electrolytebetween said electrode and a counter electrode, so redox mediatorimpurities in the electrolyte and trace electronic conductivity theelectrolyte cannot cause charge degradation. Moreover, thissupercapacitor/battery hybrid energy storage device has the optionalbenefit of being refilled with electrolyte (or the liquid component ofsaid electrolyte, such as water) at usage site—thereby providing areduced shipping weight. Though at a cost of increased device weight andvolume (needed to accommodate the weight and volume of the predominatelyfaradaic component of the hybrid device), the supercapacitor componentcan self-charge after capacitive discharge, as a result of ion andcharge flow from the predominately faradaic electrode component to thepredominately non-faradaic electrode component. The gravimetric surfacearea of the predominately non-faradaically charged electrode componentcan advantageously be at least 10 times that of the predominatelyfaradaically charged electrode component. More advantageously, thegravimetric surface area of the predominately non-faradaically chargedelectrode component can be at least about 100 times that of thepredominately faradaically charged electrode component. Variousmaterials can be used for the predominately non-faradaically chargedelectrode component and for the predominately faradaically chargedelectrode component, and these different electrode components can be thesame material having two quite different degrees of porosity.Advantageous electrode compositions for the predominately faradaicallycharged electrode compositions are alkali metals and alkali metalalloys, intercalated forms of carbon, and intercalated conductingpolymers. Porous carbon nanotube sheets are particularly suitable forthe predominately non-faradaically charged electrode composition. As forconventional batteries, there can be more than electrodes in thesupercapacitor/battery hybrid energy storage devices, and theseelectrodes can be connected either in-series or in-parallel arrangements(or a combination thereof) depending upon the required device outputvoltage. Both the flat plate and spiral would electrode arrangementsused for conventional batteries can be usefully employed for the presentsupercapacitor/battery hybrid devices.

Alternatively, from a view of optimizing discharge rate by minimizingdiffusion distances for a supercapacitor/battery hybrid energy storagedevice, the predominately faradically charged electrode component caninterpenetrate within a predominately non-faradaically charged electrodecomponent. For example, a high discharge rate and high charge storagecapacity electrode could be produced by dispersing particles ornanofibers of an doped electronically conducting organic polymer (suchas alkali-metal-doped poly(p-phenylene, a conducting polyaniline, ordoped V₂O₅ nanofibers) within a matrix comprising uncoated carbonnanofibers. Of the various alternative ways to accomplish thisdispersion, filtration of the faradaically conducting component togetherwith dispersed carbon nanotube fibers to form a composite sheet isespecially simple. Excepting the co-addition of the faradaically chargedcomponent, this is the conventional process used to form nanotube sheets(J. Liu et al., Science 280, 1253 (1998); A. G. Rinzler et al., Appl.Phys. A 67, 29 (1998)).

Devices of invention embodiments can use dynamically varied non-faradaiccharge injection to control the movement of materials through filterscontaining nanoscale and/or microscale pores. These embodiments can usepredominately non-faradaic double-layer charge injection to dynamicallyand selectively control the flow of materials (in gases, liquids, ormelts) through porous membranes.

One such invention embodiment (using the concept of discrete pores) isschematically illustrated in FIG. 17. The invention embodiments of thisfigure and that of alternative invention embodiments (where membranepores are uniformly distributed) are most generically applicable fordynamic and selective control of the permeability of porous membranesfor materials in dissolved states; particles in solutions, melt, orsupercritical states; and molten materials. In the device of FIG. 17,1701 is a filter material containing pores 1702. The walls of thesepores, which are substantially in the thickness direction of the filtermaterial 1702, are coated (at least in part) with an electronicconductor. The pictured coating on the top surface of the filtermaterial is also an electronic conductor (1701), which substantiallyelectronically contacts the electronic material on the walls of the 1702pores of the filter material. The substantially interconnectedelectronically conducting materials on the top surface of membrane 1700and the walls of pores 1702 electronically interconnects with material1700, and this total interconnected structure serves as the workingelectrode of the device. Element 1704 is the counter electrode of thedevice. This counter electrode element contacts the electrolyte 1703,but does not substantially contact the working electrode element.Element 1703 (the electrolyte) and 1704 (the counter-electrode) eitherdo not extend over the pores in the membrane, or are porousthemselves—so as not to impede the operation of the membrane. The powersource used to obtain membrane channel charging, together withelectrical leads to the electrode element 1700 and counter electrode, iselement 1704. This power source can be of variable voltage, so that themembrane filtration properties can be dynamically tuned. Discharge ofthe non-faradaically injected charge removes the ions in the pores ofthe filter, thereby enabling convenient cleaning of the filter whenneeded.

The device of FIG. 17 operates in the following way. Application of apotential between the said working electrode and the counter electrode1704 causes predominately non-faradaic charging of the electronicallyconducting material on the walls of pores 1702. This involvesessentially simultaneous charging of these walls of 1702 by electroniccharge injection and the migration of ions into the pores (preferablyfrom the electrolyte 1703) to electrostatically compensate theseelectronic charges. These ionic counter charges selectively retardtransport of materials down the pores by effectively reducing the sizeof pores 1702. Also, the effectiveness of this pore filling on transportthrough the membrane depends on the degree of charge injection, the signof charge injection versus that of components in the material beingfiltered, as well as the geometry of the ions associated with chargeinjection versus those in the material being filtered. Thesedependencies provide additional features that enable the selectivetuning of transport through the membrane by materials having differentsizes, geometries, charges, and charge distributions.

The conducting electrode material in pores 1702 need not extend alongthe entire length of the pores. In fact, a major fraction of themembrane pore volume can be used to store a drug, which is released at arate determined by the applied potential between the working electrodeand the counter electrode 1704. Also, a drug reservoir can be on oneside of the membrane so that drug delivery (for example, through theskin) is determined by membrane transport that depends upon theinter-electrode potential and the corresponding degree of non-faradaiccharging.

Devices of invention embodiments also provide electromechanicalactuation and electrochemical tunable chemical actuation. It is wellknow that charge injection, either faradaic or non-faradaic, can provideelectrochemical electromechanical actuation. This electrochemicalactuation is fundamentally different from that for magnetostrictive,electrostrictive, ferroelectric, electrostatic, and shape-memoryactuation. For examples of such faradaic and non-faradaic actuation, seeR H. Baughman, Synthetic Metals 78, 339-353 (1996); R. H. Baughman etal., Science 284, 1340 (1999); R. H. Baughman et al., U.S. Pat. No.6,555,945; G. Gu et al., Nature Materials 2, 316-319 (2003); and R. H.Baughman, A. A. Zakhidov, and W. A. de Heer, Science 297, 787-792(2002). A problem with prior art technologies of actuation usingelectrochemically-induced dimensional changes is that an electrolytemust be used, and both solid-state and liquid electrolytes providedisadvantages. First, liquid electrolytes are generally problematicbecause of the need for electrolyte containment, and on the microscale(for anything other than microfluidic applications) incompatibility withconveniently employable device fabrication methods. Second, while theuse of liquid electrolytes can avoid these problems, others appear. Mostimportantly, the mechanical modulus of the electrolyte acts to constrainthe achievable actuation in the best case, and in the worst case (forelectrolyte ion storage operation modes) the electrolytes providedimensional changes themselves and these electrolyte-induced dimensionalchanges work in opposition to the dimensional changes of at least one ofthe actuator devices. Hence, these all-solid-state electrochemicalelectromechanical actuators have been cantilever devices. Thesecantilever devices, when based on a porous actuator electrode, containelectrolyte in the volume of this electrode, as well as a separatorbetween the two needed electrodes. Hence, these prior artall-solid-state electrochemical electromechanical actuator operate bybending, thereby utilizing the mismatch in electrochemically induceddimensional changes of mechanically coupled opposite electrodes. Thispresents a problem, since such bending actuators do not provide a veryefficient way to convert electrical energy to mechanical energy. Unlikefaradaic electrochemical actuators, the non-faradaic actuators of somepreferred embodiments do not require dopant intercalation andde-intercalation during the actuator cycle, so they do not suffer fromcycle life and cycle rate limitations from such partially irreversibleprocesses.

FIG. 18 schematically illustrates an electromechanical actuator deviceof invention embodiments that operates by a different mechanism, and canprovide much larger actuation strains than any prior art device of anytype. This device uses a carbon multiwall nanotube (MWNT) thattelescopes outward in order to decrease free energy by increasing thesurface area available for charge injection. The picture on the left(elements 1800, 1802, and 1804) shows the MWNT before non-faradaiccharge injection, and the picture on the right shows the same MWNT thatis partially extended by non-faradaic charge injection. Element 1800 isthe MWNT in fully contracted state and element 1801 is the MWNT inpartially extended state. This MWNT is a working electrode for aelectrochemical cell whose counter electrode is element 1804 and 1805(which is the same element for the two indicated states of the device).The electrode elements are electronically separated by the electrolyte(designated 1802 on the left and 1803 on the right). The (−) sign onelement 1805 and the (+) sign on element 1801 indicates that a potentialhas been applied between the working electrode 1805 and the counterelectrode 1801. This applied potential results in the injection ofelectrons into the working electrode 1801, and the associated migrationof positively charged ions 1806 to the surface of this MWNT 1801.

The device operates because the actuator is a supercapacitor with anenergy associated with the capacitance C of E_(c)=½ q²/C, where q is theamount of charge injection. Since charge injection is largely tonanotube walls that are externally exposed (rather than internal to themultiwall nanotube), the capacitance C of the multiwall carbon nanotube(MWNT) increases approximately proportionally to the external wall areaof the MWNT (A_(e)). Hence, C=A_(e)C_(a), where C_(a) is the capacitanceper exposed surface area of the MWNT. At fixed degree of chargeinjection q, C can be decreased by increasing A_(e) by thetelescope-like extension of the MWNT. This provides the driving forcefor actuation, which is opposed by the energy cost of creating newexternal surface area.

A device of FIG. 18 can be fabricated, for example, using methodsalready employed for making carbon nanotube tips for atomic forcemicroscopy (AFM) and scanning tunneling microscopy (STM). For example, aMWNT can be grown on the tip of a conducting doped silicon nanoprobetip, which provides electrical connection to the MWNT electrode.Thereafter, the electrolyte layer can be deposited on the Si nanoprobetip either by electrochemical polymerization, chemical reaction, orsolution deposition. Then a conducting polymer layer can be deposited ontop of the electrolyte layer by chemical reaction or solutiondeposition. This conducting polymer layer serves as the counterelectrode of the device. Thereafter, the electrolyte coating and theconducting polymer coating over most (but not all) of the MWNT can beremoved by conventionally employed methods, such as the application of avoltage pulse. Finally, a nanoprobe manipulator can be used to makecontact with the conducting polymer counter electrode. The therebyobtained MWNT electromechanical actuator can be used for manipulationson the nanoscale. This actuating nanoprobe tip can be used in gaseous,vacuum, or liquid environments, including biological fluids.

The mechanical actuators of the present invention embodiments can be runin reverse to convert mechanical energy to electrical energy formechanical sensor and energy conversion devices. The benefits of usingelectrode elements that are not in direct contact with the electrolyteis the same as for the above-described actuators that are used toconvert electrical energy to mechanical energy. For cases in which theworking and counter electrodes are identical, the generation ofelectrical energy requires that mechanical stress be applied differentlyto these electrodes. Typically, a tensile stress is applied to oneelectrode while a compressive stress (or a decrease in tensile stress)is applied to the other electrode. These electrochemical devices forsensing mechanical stress and strain and for converting mechanicalenergy to electrical energy generate high currents at low voltages,which provides advantages for some applications over ferroelectricmechanical-to-electrical energy converters, which generate low currentsat high voltages. This performance of electrochemical energy convertersof the present invention embodiments is desirable for minimizing theeffect of lead capacitances for remotely located sensors, so thatsensor-response amplifiers need not be located down-hole when doingseismology for oil exploration. The ability to operate theseenergy-harvesting devices at low frequencies could be usefully exploitedfor the conversion of mechanical energy of ocean waves to electricalenergy. An array of such devices can be electrically interconnected inseries to provide an increased output voltage. In another embodiment,the electrodes of the electrochemical electromechanicalenergy-harvesting device are electrically biased during device operationusing an applied voltage. An advantage of such biasing is that theelectrical energy generated by mechanical stress can be increased.However, when using such biasing, the stress generated voltage changesshould ideally be electrically isolated from the biasing voltage. Thiscan be accomplished by using a capacitor in series with the biascircuit. These devices used to convert mechanical energy to electricalenergy typically utilize either a uniaxial or biaxial applied stress.Device polarity can be achieved using different materials for opposingelectrodes, an applied bias voltage, or the application of differingstresses to the opposing electrodes.

FIG. 19 is illustrative of one generically applicable deviceconfiguration of invention embodiments. This device is a nanoprobe thatcan be used for such diverse applications as atomic probe imaging andelectron emission. Element 1900 is a conducting nanoprobe tip base (suchas an elemental metal), which mechanically supports the device andprovides electronic connection to element 1903, which is anelectrochemical counter electrode for the device. Element 1901 is anelectrolyte that separates the counter-electrode element 1903 fromelement 1902, which is part of the working electrode. Element 1904 is ananofiber, such as a carbon single wall nanotube, a multiwall carbonnanotube, or a bundle of these nanotubes that electronicallyinterconnects only with element 1902. Element 1905 is a power source(and associated electronic interconnects) that provides an adjustablepotential between the counter electrode 1903 and the working electrode,which comprises elements 1902 and 1904.

In the operation of this device, a change in the potential appliedbetween working electrode and counter electrode (by power source element1905) causes a change in the degree of electronic charge injection inelement 1904, as well as the migration of ions on element 1904 tocompensate these electronically injected charges.

Consider first the application of the device of FIG. 19 for scanningprobe imaging, such as either atomic force microscopy or atomictunneling microscopy. The dynamically controllable charge injection into1904 (via the potential applied by 1905) changes the work function of1904 and therefore can dynamically tune the properties of this probe tipfor atomic tunneling microscopy. Equally important, and sometimes moreimportant for some applications, the device of FIG. 19 is configured sothat electrochemical charge injection in nanotube tip 1904 isaccompanied by the transport of counter ions onto the nanotube tip 1904.These ions can substantially modify in a tunable way the tunnelingcharacteristics of the nanotube tip 1904 for both atomic force andatomic tunneling microscopes. Use of the device of FIG. 19 as a fieldemission tip can utilize both the charge in work function of the tip (asa result of electronic charge injection) and the effect or counter ionson this tip on field emission. Moreover, the device of FIG. 19 can bejust one of an array of like field emission tips that can beindependently controlled electronically.

The devices like shown in FIG. 19 can be constructed by applyingpresently know technologies for nanofabrication. For example, thenanoprobe base 1900 (which is an electrically conducting contact) can becoated in the tip region by the counter electrode material 1903, whichcan be a conducting organic polymer that is deposited from polymersolution (or deposited from monomer solution by electrochemicalpolymerization). The electrolyte 1901 can be then over coated on thecounter electrode element (again, for example, by either solutiondeposition or electro-polymerization), as can be the subsequent coatingof element 1902, which is part of the working electrode. The nanotubetip 1904 can be attached to the electronically and ionically conductingelement 1902 by various methods. For example, until the electrolyte 1902loses the solvent used for deposition, this layer can act as an adhesivefor harvesting a nanotube or small nanotube bundle from apre-synthesized nanotube array, such as a nanotube forest. Electricalcontact to the counter-electrode 1903 is by the conducting nanoprobebase 1900. The other electrical contact (to the working electrodecomponent 1902 can be made using a nanomanipulator during an inspectionprocess to assess whether or not the nanotubes element 1904 is correctlyadhering to 1902. For the purpose of conveniently making said contact(as well as for convenience in making suitable depositions on 1901 and1902) it should be understood that these layers can extend much furtherback from the tip of 190 than is shown in FIG. 19.

FIG. 20 shows a device that operates like the device of FIG. 19, exceptthat the nanotube probe tip base makes electrical contact to thenanotube. Such design facilitates device construction. The nanoprobebase is the conducting cylinder 2000, the power source with associatedelectrical interconnects is 2005, the counter-electrode is 2001 (whichcan optionally be a doped conducting polymer), the electrolyteseparating working and counter electrodes is 2002, the working electrodeis comprised of 2003 (which can optionally be a conducting polymer) andelement 2004. Although element 2004 is shown here as a carbon nanotube,various conducting probe tip materials can serve this function. In fact,element 2004 can be just a sharpened part of the nanoprobe base 2000,such as an electrochemically-thinned tungsten wire. Also, element 2003can optionally be eliminated.

As an example, the fabrication of probes of the type shown in FIG. 20,is described, but with element 2003 eliminated (so diffusion of ionsalong the thereby revealed tip surface of the nanoprobe base 2000 isrequired for electrochemical charging of 2004 during device operation.The nanoprobe base (2000) with attached nanotube 2004 is commerciallyproduced, and these are typically made by depositing catalyst on the tipof nanoprobe base 2000, and using chemical vapor deposition to grow thenanotube on 2004 on 2000. (For several methods that can be used for suchnanotube growth on a probe tip, see J. H. Hafner et al., Nature 398,761-762 (1999) and J. H. Hafner et al., J. Physical Chem. 105, 743-746(2001)). After making electrical connection to nanotube base 2000,nanotube base 2000 and attached 2004 can be dipped into an electrolyte,so that an insulating protective coating can be deposited onto the tipof nanoprobe base 2000 and all of element 2004. (See J. K. Campbell etal., J. Am. Chem. Soc. 121, 3779-3780 (1999) for methods for monitoringthe depth of immersion into the electrolyte, so only the tip region iscoated with an insulating protective coating.) The electrolyte 2002 canbe then electrochemically polymerized onto the desired region of thedevice (since the insulating character of the protective coating willprohibit deposition on top of this coating. Next, element 2001 can bedeposited from solution (via a dip/dry process) and electricalconnection is made to element 2001. Deposition of the material ofelement 2001 on top of the protective coating is avoided by havingselected the coating material so that it is not wet by the mentionedsolution. Finally, either chemical etching or dissolution in a solventremoves the protective coating.

FIG. 21 illustrates an invention embodiment in which a high-surface-areananostructured material (2110) functions as an electrochemically gatedion beam source. This device uses our surprising experiment observationthat we can non-faradaically inject charge in a nanostructured element,and retain this charge in the absence of a contacting electrolyte. Whilethis figure pictures a carbon nanotube as the emission element (2110),this emissive element can be any of a variety of nanostructuredmaterials whose surfaces (interior, exterior, or a combination ofinterior and exterior) can contain counter ions that can be fieldemitted. Element 2101 is an electrical insulator. Element 2102 is avacuum or other medium in which the source of field emission species(2103) are dispersed. For example, this medium can be liquid, gaseous,or solid (such as an alkali metal that is evaporated into the gaseousstate using an heater; a solution of an alkali metal, or a complex of anorganic solid with an alkali metal). Element 2100 in this figure servesas an electronically conducting electrode contact, as well as aconfinement wall for 2102 and 2103 (although the containment andelectrode contact element of 2100 can optionally be separate materials).The field emission precursor species (element 2103) can be eithercharged or neutral. If this species is neutral, should be capable ofundergoing at least partial charge transfer with counter electrodeelement material 21004 for some applied potential to this element. Ifthis field emission species is charged, with the counter ion to thischarge residing in 2102, counter-electrode element 2104 is usefullycapable of undergoing charge transfer with this counter ion charge. Forexample, a neutral species 2103 can be an alkali metal atom thatundergoes oxidation or reduction with counter electrode element2104—thereby simultaneously transferring electronic charge to 2104, andbeing converted to an ion. If electron emission species 2103 is charged,the counter-ion to this charge within 2102 should ideally be able oftransferring electronic charge to counter-electrode element 2104,thereby enabling the migration of the ion emission species into 2104.The electron emission species 2103 becomes the ionic electron emissionspecies 2105 within counter-electrode 2104, as well as the same ionemission species that is pictured within electrolyte 2107, the workingelectrode components 2108 and 2110, and the field emitted ion beam 2111.The counter charges to the ionic emission species in 2104, 2108, and2110, are, respectively, the electronically injected charge (holes orelectrons) in these elements. The counter-ion species in electrolyte2106 to the type ions found in 2105 are the counter ions 2106 foundwithin 2107. Element 2112 is the electrostatic counter electrode to theion emission source 2110, wherein 2112 is at a negative potential withrespect to 2110 if the ions emitted by the ion source are positive ions.Elements 2113 and 2114 are voltage sources (and associated electronicinterconnects), which are able to apply a variable voltage. The voltagesupply element 2113 applies a voltage between the electrode (comprising2108 and 2110) and the counter-electrode element comprising 2104 (alongwith the associated ions 2104 and 2106) via the electrode conductingelements 2100 and 2109.

The operation of the device of FIG. 21 is described in the following.Element 2100 helps confine a source of material for ion emission (2103),which can be in either charged or neutral. Either as a result of (1)electronic charge transfer from neutral forms of this field emissionspecies to counter-electrode element 2104 or (2) electronic chargetransfer from counter ions (within 2102) to counter-electrode element2104, the ion emission species derived from 2103 are able to migrateinto the counter-electrode element 2104 (where they become 2105). In theabsence of parasitic reaction processes, migration of the ion emissionspecies 2105 across the electrolyte 2107 and into working. electrodecomponents 2008 and 2110 is controlled by the potential applied by 2113.The application of a potential with suitable sign and magnitude between2110 and 2112 causes the emission of the ion beam 2111.

The preceding figures showing invention embodiments have used electricalwires to connect electrochemical electrodes to the power source. FIG. 22shows an invention embodiment in which electrochemical charge injectionresults from charging via an electron beam, so there is no directcontact of an electrical lead to the electrochemically charged element.The major advantage of such electron-beam-induced charging is that theelectron beam serves as an electrical contact that can be convenientlychanged at will and, more importantly, used to individually addresselectrode elements (called pixels) that are on the nanometer orsub-nanometer scale. Interestingly, electron beam irradiation can beused to charge nanoscale pixels either negatively or positively—lowelectron energies result in negative charging, and high electronenergies result in positive charging (since the loss of secondaryelectrons can exceed the gain of electrons from the primary beam).

FIG. 22 provides a schematic cross-sectional view of a device in whichindividual nanoscale pixels can be electrochemically charged eitherpositively or negatively using a focused electron beam. Element 2203comprises a electronically conducting support 2200, electronicallynon-interconnecting wire-like pixels 2202 and 2210 that are a parallelarray, and a solid-state electrolyte 2201 that interpenetrates the pixelarray. Element 2204 is an enclosure that separates the vacuum (2205) onthe inside of 2204 from both the ambient atmosphere and an atmospherethat can be arbitrarily chosen for the enclosed region 2209, whichcontains samples that are placed on top of the exposed pixels 2210.Element 2211 is a voltage source that provides the electron-beam-inducedelectrochemical charging of pixel elements like 2202 and 2210. Theelectron beam, used for charging, is 2206. This focused electron beam isproduced by the electron beam source 2207. Sheet 2208, which completesenclosure 2209, can optionally be used as an easily removable entry portfor placing samples in contact with pixels 2210 within enclosure 2209.

The device of FIG. 22 operates as follows. Charging of a particularpixel in the pixel array is accomplished by focusing the electron beam2206 on that pixel. Depending upon the voltage used to produce theelectron beam, the nanotube becomes either negatively or positivelycharged. For example, if electrons are injected into a particularnanotube, holes will be injected into the counter-electrode pixels(i.e., those contacting 2200). The existence of electrolyte 2201 enablesthe charging of individual nanotubes to much higher extents than wouldbe possible if 2201 were a dielectric, since ions from electrolyte 2201can locally electrostatically compensate for the electronic charge onthe pixels.

The device of FIG. 22 can be used for various usefulprocesses—including, for example, the assembly of nanoscale objects onthe platform comprising the pixel array. The assembly process utilizedhere is the switching of surface tension of a pixel as a result ofpredominately non-faradaic electrochemical charging of the pixel. Thethereby self-assembled nanostructured objects, which could be thecomponents for a nanoscale circuit board, can optionally be transferredto another substrate for subsequent fabrication steps. This process ofpixel-based assembly and transfer of assembled nanoscale objects can berepeated as needed for a manufacturing process. Surface tension changesinduced by temperature gradients, selective area deposition, or byphoto-induced reactions are well known in the prior art. However, thisuse of electron-beam-induced pixel charging for electrochemicallyswitching surface energy is unique to the present invention embodiment.

In another application mode, the device of FIG. 22 is used for a newtype of nanoscale microscopy, which we call Electron-BeamElectrochemical microscopy (EBE microscopy). This application made usesthe well-known fact that detection of scattered electrons can be used tomeasure the potential of an element in an electron microscope. While thechoice of investigated material is virtually unlimited, biologicalmaterials (like cells and cells arrays and DNA) provide importantapplication opportunities. The microscopy uses oppositely charges pixelsas opposite electrodes for doing spatially resolved electrochemicalcharacterization, including spatially resolved cyclic voltammetry(curves of current versus potential at constant voltage scan rate, orrelated curves of potential versus charge at constant current). Suchspatial information on the oxidation and reduction processes can be usedto probe chemicals within cells or to sequence DNA. The device of FIG.22 employs focused electron beam generation and electron energydetection capabilities that are well know, and widely used in scanningelectron microscopes and scanning transmission microscopes. Numerouspapers in the prior art describe methods for making vertically alignednanotube arrays (nanotube forests) and manipulating these nanotubeforests to make devices. These methods can be usefully employed for thepresent invention embodiments that use nanotube forests. Examples ofthese methods are described by P. Soundarrajan et al. in J. Vac. Sci.Technology A 21, 1198-1201 (2003), J. Li et al., J. Phys. Chem B 106,9299-9305 (2002), and A. M. Cassell et al., Nanotechnology B15, 9-15(2004).

The discovery that electrochemically injected charge is stable in theabsence of electrolyte also enables the dynamic tuning of nanostructuredmaterials properties for materials absorption and desorption, such ashydrogen storage. Charge injection (and associated ion migration)selectively changes the interaction energy of materials with ananostructured substrate (such as nanostructured fibers, sheets, andpowders) and therefore changes the absorption capabilities of thesubstrate material. Changing the degree of charge injection by changingthe applied potential can aid in the release of absorbed materials.

Invention embodiments also provide the use of predominately non-faradaicelectrochemical charge injection for dynamically tuning thermalconductivity. These invention embodiments use the unexpectedobservations described herein that non-faradaically injected electroniccharge and associated counter ions are stable on nanostructuredmaterials even in the absence of a contacting electrolyte. Use is alsomade here of the fact that phonon scattering lengths are long inlow-dimensional nanostructured materials (like carbon nanotubes), sothese materials can have large thermal conductivities (see P. Kim etal., Phys. Rev. Lett. 87, 215502-1 to 215502-4 (2001)). The largethermal conductivities can be usefully employed, for example, forconnecting hot circuit-board components to a material that is at lowertemperature. However, for both the purpose of regulating temperature andavoiding the heating effect of electrical pulses sometimes used inthermoelectric cooling, it is desirable to be able to electrically tunethermal conductivity. This tunability of thermal conductivity isachieved here as a result of the combined effects of electronic chargeinjection and the associated migration of counter ions on the surface ofthe nanostructured material (which can change thermal conductivitybecause of a change in charge carrier density, can increase conductivitybecause of phonon transport in the ion layer, and can decrease thermalconductivity because of ion-based scattering of phonons associated witha highly thermally conducting nanostructured material, like carbonnanotubes).

FIG. 23 schematically illustrates one type of device of inventionembodiments that provides tunable thermal conductivity. The blocks inFIG. 23 (2300, 2302, and like elements) are arrays of multiwallnanotubes that have been grown on a substrate by patterned deposition ofcatalyst on the substrate, followed by growth of a nanotube forest(comprising parallel nanotubes that are orthogonal to the substrate).While this sample is not yet infiltrated with electrolyte, 2301 showsthe inter-block spaces where electrolyte can be infiltrated duringdevice fabrication. The blocks of nanotubes in FIG. 23 are 100 micronsin lateral dimensions, although either larger or smaller dimensions canbe employed for these blocks. An advantage of using small lateraldimensions is in an enhanced rate of tunability, and a disadvantage isan increased cost of device fabrication. The substrate (upon which theseblocks rest) contains conducting pads, so alternating blocks provideworking and counter-electrodes for an electrochemical cell. Theapplication of a potential between these working and counter electrodescauses charge injection and associated ion injection into the nanoporousblocks, and thereby charges thermal conductivity. This changed thermalconductivity varies in an electronically controllable manner thermaltransport along the nanotubes between the substrate and a material thatis positioned on top of the blocks of nanotubes.

The electrolyte 2301 can fill or partially fill the regions between thenanotube blocks in FIG. 23. It is required that electrolyte connects atleast one working electrode bock with one counter electrode block.Pervasive presence of the electrolyte 2301 in the structure isproblematic, since the extensive presence of this electrolyte can act toreduce the dynamic range of thermal conductivity tuning (since theelectrolyte can also contribute to thermal conductivity). When a largedynamic tunability range is needed (albeit with some sacrifice totunability rate), the electrolyte should preferably not substantiallyinfiltrate the blocks (2300, 2302, and like blocks). From this view ofmaximizing dynamic range of tunability, the electrolyte can optionallybe deposited so as to interconnect blocks in only one direction. Also,the electrolyte need not fill the cavity between any two adjacentblocks. While this invention embodiment is illustrated using an array ofidentical nanotube blocks, non-block arrays can also be used and thematerial used for electrodes need not be comprised of carbon nanotubes.Also, the material used for working and counter-electrode elements canbe different. In addition, the counter electrode element need not beporous and need not be charged predominately non-faradaically duringdevice operation. For example, the working electrode elements can be ametal mesh made by a known opal templating process (see A. Zakhidov etal. in Science 282, 897 (1998), U.S. Pat. No. 6,261,469, and U.S. Pat.No. 6,517,762, as well as O. D. Velev et al. in Nature 401, 548 (1999))and the counter-electrode element can be a doped conducting polymer.Another useful method for making nanoporous metals for such deviceembodiments is by dealloying alloys (see J. Erlebacker et al., Nature410, 450-453 (2001)).

FIG. 24 illustrates an invention embodiment in which predominatelynon-faradaic charge injection is used to optimize the figure of merit(ZT) of thermoelectric elements (2402 and 2405). These thermoelectricelements are interconnected by electrolyte 2401. The indicated appliedpotentials on electrodes 2400, 2403, 2404, and 2406 results inpredominately non-faradaic charge injection into these electrodes(electrons in 2402 and holes in 2405), which optimizes the ZT of 2402and 2405, by changing the thermal power (S), thermal conductivity(σ_(T)) and electrical conductivity (σ_(e)) of these elements (ZT, whichis an efficiency index, is S²σ_(e)/σ_(T)). These thermoelectric elementscan be interconnected electronically like for conventionalthermoelectric devices to provide either a cooling capability (picturedin FIG. 24), or the capability of generating electrical energy from atemperature difference. In these cases, the top electrodes (2400 and2404) are at a different temperature than the bottom electrodes (2403and 2406) during device operation.

At least one of the electrodes 2405 and 2402 should be porous. In theillustrated case, both of these electrodes are porous, and made by theopal templating process of A. Zakhidov et al., Science 282, 897 (1998)and U.S. Pat. No. 6,261,469 and U.S. Pat. No. 6,517,762). A directphotonic crystal (also called a direct-lattice photonic crystal or aopal photonic crystal) typically comprises spheres. An inverse photoniccrystal (also called an inverse-lattice photonic crystal or inverse-opalphotonic crystal) is made by templating the void space of a directphotonic crystal. Like for the device of FIG. 23, there are advantagesin minimizing the amount of electrolyte in the device. The reason fordoing so is that any electrolyte that provides a substantial path forthermal transport between hot and cold regions of the device can degradeZT, since it increases the effective σ_(T).

The unexpected discoveries involving this invention show that it is notnecessary for electrolyte to substantially fill nanoporous electrode ofFIG. 24 (2402 or 2405) in order to obtain electrochemical tunability ofthermoelectric properties. This is in contrast with the prior art, suchas earlier teachings of R. H. Baughman et al. in U.S. Pat. No.6,555,945. The benefit of largely eliminating the electrolyte from theseporous elements is to decrease the effective contribution of electrolytethermal conductivity to σ_(T) in the equation for ZT. In fact, ZT can bemaximized by using the minimal amount of electrolyte that is needed toprovide a continuous ion path between the working and counter electrodes(2402 and 2405).

Devices of invention embodiments can be used for the control ofelectromagnetic wave propagation and reflectivity for ultraviolet,visible, infrared, radio frequency, and microwave frequency regions.Such switching can use changes in electrical conductivity, refractiveindex, dielectric constant, absorption, and reflectivity as aconsequence of electronically controlled predominately no-faradaiccharge injection. It is known in the prior art that such changes can beinduced in porous materials by electrochemical non-faradaic chargeinjection. However, the prior art has not understood that suchelectrochemical switching of properties can occur for material regionsthat are not contacted by an electrolyte. One benefit of inventionembodiments is the decrease or total elimination of undesirableproperties contributions from an electrolyte.

The sensor device of FIG. 13 provides one invention embodiment in whichoptical properties are tuned without employing electrolyte contact totuned regions that are accessible to the electromagnetic radiation. Thisembodiment utilizes an electrolyte that is on the reverse side of theoptical element (and obscured by it), so that presence of theelectrolyte did not adversely affect achieved properties. The device ofFIG. 25 is one in which the electromagnetic radiation propagates eitherat least approximately parallel to the electrode layers or predominatelyalong the lengths of element 2500 (and like elements), along the lengthsof element 2502 (and like elements), or along both of these types ofelements (in contrast with the possibly approximately perpendicularpropagation of the electromagnetic radiation for the device of FIG. 13).Element 2500 is the negatively charge-injected electrode and 2501 is anelectrical contact for this electrode. Element 2502 is a positivelycharge-injected electrode, element 2503 is an electrical contact to thiselectrode, and element 2504 is an electrolyte that separates elements ofthe type 2500 from elements of the type 2502. Charge injection in 2500and in 2502 (and like elements in the array) provides the desired chargein transmission and reflection properties of the device. The deviceoperates by the changes in properties for electromagnetic wavepropagation that result from charge injection into 2500 and 2502 (andlike elements). The benefit of this device arrangement is that theelectrolyte 254 need not penetrate into the elements (2500 and 2502)that are switched by charge injection, so the effects of chargeinjection on electromagnetic wave propagation are not obscured bypossible absorption of electromagnetic radiation by 2504. These benefitsresult from the discovery of the inventors that predominatelynon-faradaic charge injection can occur without the need for contactingelectrolyte.

FIG. 26 schematically illustrates an EBIG (Electrolyte-Bare Ion Gated)device that provides electroluminescence by using a semiconductingnanostructured material—in this case a semiconducting carbon nanotube.Elements 2600 and 2601 are electrical contacts to a semiconductingnanotube, which are charged negatively and positively, respectively, bythe battery and associated electrical connections that comprise element2604. Element 2602 is an electrolyte that provides a continuous pathbetween opposite ends of the nanotube (element 2603). This deviceoperates by hole injection on one end of the nanotube and electroninjection on the opposite nanotube end. These injected charges arecompensated by ions of opposite sign that migrate along the nanotube tocoulombically compensate for the injected charge. Light (indicated bythe arrow 2605 is emitted as a result of recombination of injected holesand electrons. Unlike other invention embodiments, the working andcounter electrodes of this device are electronically interconnected,albeit with a semiconducting nanotube.

FIG. 27 schematically illustrates another EBIG that is light emitting,wherein the light-emitting element is a semiconducting inverse-opalphotonic crystal (2700). Elements 2702 and 2703 are source and drainelectrodes, respectively, and are simultaneously working and counterelectrodes. These electrodes contact opposite sides of the picturedphotonic crystal 2700, which is also contacted by the electrolyte 2701,which can be a solid-state electrolyte that is highly transparent in thespectral range where light emission is required. Element 2704 is aninsulating substrate material, which can optionally contain an array ofthe EBIG light emitters, as well as semiconductor devices forindependently varying light emission from each EBIG light emitter. Holeand electrons are injected on opposite ends of the semiconductingcrystal 2700 as a result of the EBIG gating, which are electrostaticallycompensated by the diffusion of counter-ions from the electrolyte 2701.Charge carriers of opposite sign are injected by the source and drainelectrodes and their recombination in 2700 results in the emission oflight (illustrated by element 2705). The benefit of having theelectrolyte substantially non-penetrating in 2700 is that the width ofthe photonic bandgap is unaffected by the refractive index of theelectrolyte (although it can be effected to a lesser extent by ions thatdiffuse into 2700 during electrochemical gating). The benefit of using aphotonic crystal as the light-emitting element is that the emission fromthe photonic crystal is directional. Also, by using teachings of theprior art for non-gated photonic crystal lasers, light emission bylasing can be obtained.

FIG. 28 schematically illustrates a back-gated electrolyte-bare ion gate(EBIG) field effect transistor that has an air gap. This device differsfrom air-gap field-effect transistors in that the electrolyte 2804 ispresent, which can supply counter ions of opposite sign for electroniccharges injected into channel 2800 by the source electrode (2801) anddrain electrode (2802). Element 2805 is the gate electrode, element 2806is an insulator, and element 2803 is an air gap. For application modesin which the device of FIG. 28 is used like a Chem-FET (field effecttransistor chemical sensor) for gas sensing, this air gap can optionallybe used as a gas flow path for gas sensing and analysis. As for otherEBIG sensor devices of the invention embodiments, devices of FIG. 28 canbe used for detailed gas sensing by the equivalent of gas cyclicvoltammetry, in which case it is useful to have one or more referenceelectrode in electrolyte 2804.

This device of FIG. 28 fundamentally differs from the ion-gated devicesof the prior art in that the channel electrode 2800 is not fullycontacted with electrolyte and non-contacted regions substantiallycontribute to device functionality. The entire device of FIG. 28 canoptionally be encapsulated in a protective material, such as a barrierpolymer, using known art of device coating. Various device geometriesorthogonal to this cross-sectional view can be conveniently used. Forexample, application modes of this device as a gas sensor can profitablyuse a configuration in which the air gap 2803 is used for gas deliveryand is elongated orthogonal to the plane of the drawing. On the otherhand, using either a square or cylindrical geometry for this air gap canfacilitate miniaturization. The left and right parts of channel 2800(together, respectively, with the contacting electrodes 2801 and 2802)can be viewed as two current-carrying electrochemical electrodes.Element 2805 is a third current-carrying electrochemical electrode.Charge injection into the left and right sides of the channel will beidentical if the potential difference between 2801 and 2802 isnegligible compared with that between 2801 and 2805. In the case where2805 is electronically floating, a change in the potential between 2801and 2802 will result in hole injection in one side of the channel 2800and hole injection in the opposite side of 2800. This channel ideallyhas a semiconducting state, so that changes in the degree of chargeinjection can provide large changes in the conductivity of the channel,which is measured by the source-drain current (between 2801 and 2802)that results from the application of a small change of potential betweensource and drain. The channel 2800 can be either nonporous or solid, andelement 2805 can be either faradaically or non-faradaically chargedduring device operation. Also, ions can either be stored in theelectrolyte in one state of the device or these ions used for thecharge-discharge process of 2800 and 2805 can be shuttled between 2805and 2800 (and the reverse) during device operation. A combination ofthese ion processes can also be profitably used. In one deviceembodiment, the gate 2805 is optionally a metallic conductor. In anotherinvention embodiment, 2805 is a semiconducting material, electricalcontacts are provided between opposite ends of 2805, and measurement ofcurrent flow between these electrical contacts (in response to apotential difference between these contacts) provides a second type ofdevice response.

Various metal disulfides are especially useful as channels for thedevices like that shown in FIG. 28, like the semiconducting WSe₂, MoS₂,MoSe₂, HfS₂, SnS₂ layer phases. The related phase of NbSe₂ is useful forother invention embodiments, since it is metallic at ambient andsuperconducting at low temperatures.

Note that the device configuration of FIG. 28 can also be used for anelectroluminescent light source if element 2800 is a semiconductor. Inthis case, electrode 2805 is optional and functions to assist inelectrochemical charge injection in the light-emitting element 2800. Theconnecting electrolyte path between the left and right sides of thechannel 2800 need not be present. In such case, transport of ionsbetween opposite ends of 2800 can occur either within 2805 or on thesurface of 2805.

The device of FIG. 29 shows the advantageous combination of non-faradaicelectrochemical and gas-gap-based electrostatic charge injection for achemical sensor that can replace conventional Chem-FETs. This example isfor a Chem-FET sensor device that is at the same time a EBIG sensor. Ananalyte gas is passed through conduit 2906. Element 2909 is either anair gap or, if elements 2906 and 2909 are optionally connected, a gapthat is filled with the analyte gas. Element 2907 is an electrolyte;element 2903 is a semiconducting channel; elements 2904, 2905, and 2910are source, drain, and gate electrodes for dry-state electrochemicalcharge injection; element 2908 is an insulator; and elements 2901, 2902,and 2900 are, respectively, source, drain, and gate electrodes for thegas-gap-based electrostatic charge injection. The benefit of this hybridChem-FET is that either dielectric-based or electrochemical dry-statecharge injection can be used (or a combination thereof). Only theelectrochemical charge injection results in the movement of ions ontothe channel, and these ions modify device response for a given degree ofcharge injection. Hence, the availability of these two charge injectionmethods enables improved selectivity for analyte detection.

The technologies for fabricating nanoscale materials, nanoscale devicesand nanoscale materials assembled within macroscopic devices haverapidly advanced in the last decade, and these technologies can beexploited for the fabrication of devices of invention embodiments. Forexample, numerous methods have been developed that are applicable forpositioning nanofibers within devices, orienting these nanofibers indesired directions, and providing appropriate electronicinterconnections. Issues such as determining the most appropriatecontacting metal and the deposition methods applicable for these metalsin nanoscale devices are resolved to a satisfactory state in the priorart literature (see, for example, Y. F. Hsiou et al., Applied PhysicsLetters 84, 984-986 (2004) for methods for applying low resistancecontacts to carbon nanotubes).

Well known and widely practiced micro-lithographic techniques can beused to obtain patterned depositions and to anchor nanofibers (such ascarbon nanotubes) on a substrate. A useful solution to the problem offirmly mechanically and electrically contacting individual single-wallnanotubes or single-wall nanotubes bundles (while leaving much of thenanotube or nanotube bundle free of constraining contacts) has beendescribed by D. A. Walters et al. in Applied Physics Letters 74,3803-3805 (1999). Using micro-lithographic techniques, these authorsdemonstrated that single-wall nanotubes could be rigidly attached onopposite ends (both electrically and mechanically), so that they aresuspended over a microscopic trench.

Various other methods well known in the prior art can be used to providenanofibers that are positioned in desired locations on substrates, andappropriately oriented. One method is by “scanning tip electrospinning”,which has been described by J. Kameoka et al. in Nanotechnology 14,1124-1129 (2003). This method achieves patterned deposition ofcontinuous individual nanotube fibers by electrostatic spinning from amicrofabricated spinning tip. Electrostatic spinning is typically of asolution of a polymer (such as polyethylene oxide, polyacrylonitrile,DNA, a conducting polymer like polyaniline, polyethylene, and variouscopolymers), which can optionally include particles such as carbonnanotube fibers (see D. H. Reneker and I. Chun in Nanotechnology 7,216-223 (1996)). Depending upon the final state needed for the patternedspun nanofibers, the polymer can be pyrolized (such as to convert apolyacrylonitrile nanofiber to a carbon nanofiber), dissolved, orchemically etched array (so as to reveal the nanoparticles, such ascarbon nanotube fibers). Probably the most commonly used method forstarting nanotube growth from a desired position on a substrate is todeposit the catalyst nanoparticles used for growth at that position, forexample using either contact printing or photo-lithographic methods (H.Dai, Accounts Chemical Research 35, 1035-1044 (2002)). As shown in thepreceding reference, contact printing on posts within a post array canbe used to grow nanotubes between these posts—like the wires spanninghigh-tension towers. Orientation of nanofibers in a desired direction onsubstrates can involve such methods as dielectrophoresis (R. Krupke etal., Nano Letters 3, 1019-1023 (2003)), the orientating effects ofliquid crystals or fluid flow fields (S. Huang at al., J. Am. Chem. Soc.125, 5636 (2003)), and magnetic or electric fields (Y. Zhang et al.,Applied Physics Letters 79, 3155-3157 (2001) and H. Dai, AccountsChemical Research 35, 1035-1044 (2002)). Also important for devicefabrication, nanofibers (such as carbon nanotubes) self-orient duringmicrowave plasma-enhanced chemical vapor growth to be locallyperpendicular to the growth substrate even when this substrate is highlynon-planar, such as the surface of an optical fiber (see C. Bower etal., Applied Physics Letters 77, 830-832 (2000)). Like the case ofcarbon nanotubes, ZnO conveniently self-organizes into usefulforest-like arrays (with the nanowires orthogonal to the substrate)during simple vapor transport and condensation processes (M. H. Huang etal., Science 292, 1897-1899 (2001)). Forms of elemental carbon,especially carbon nanofibers or graphite are included in somecompositions as one or more electrodes for invention embodiments. Likefor the case of conducting polymers most of these carbon compositionscan be used as either predominately non-faradaic or predominatelyfaradaic electrodes, depending upon the gravimetric surface area of theform of carbon and the potential of device operation. For use as anon-faradaic electrode or non-faradaic electrode component, graphite isespecially suited to be in the form of largely exfoliated graphite.Suitable carbon fibers include multi-wall nanotubes (which compriseconcentric graphite sheets), single-wall nanotubes (which comprise asingle cylindrical graphite sheet), carbon fiber scrolls (a spirallywound graphite sheet), and carbon fibers with radial alignment (in whichgraphite planes extend radially about the fiber direction). Sincemulti-wall carbon nanotubes, single-wall carbon nanotubes (SWNTs), andcarbon nanotube scrolls have hollow interiors, these hollow materialscan optionally be filled by insulating, semiconducting, or metalliccompositions by known methods, so as to thereby modify charge injectioncharacteristics. In order to maximize surface area for predominatelynon-faradaic charging, the number-average diameter of single-wall andmulti-wall carbon nanotubes used as predominately non-faradaicelectrodes is preferably below about 10 nm. The term number-averagediameter means the ordinary average of the diameters of the nanotubes,without any special weighting according to the size of the diameter.

Single-wall carbon nanotubes (SWNTs) are especially well-suited for useas electrodes. The dual laser method, the chemical vapor deposition(CVD) method, and the carbon-arc method are suitable methods for makingthe carbon nanotubes, especially single-wall carbon nanotubes, and thesemethods are well known in the literature (R. G. Ding et al., Journal ofNanoscience and Nanotechnology 1, 7 (2001) and J. Liu et al., MRSBulletin 29, 244-250 (2004)). Carbon single-wall nanotubes can havearmchair, zig-zag, or chiral arrangements of carbon atoms. Thesenanotubes are differentiated in that the armchair nanotubes have acircumference of para-connected hexagonal rings (like found inpoly(p-phenylene)), the zig-zag nanotubes have a circumference oflinearly side-connected hexagonal rings (like found in linear acenes),and the chiral nanotubes differ from the armchair and zig-zag nanotubesin that they have a sense of handedness. The geometry of carbonnanotubes is specified using conventional nomenclature using the indices(n, m). Depending on the appearance of a belt of carbon bonds around thenanotube diameter, the nanotube is the armchair (n=m), zig-zag (n orm=0), or chiral (any other n and m) variety. All armchair SWNTs aremetals; those with n−m=3k, where k is a nonzero integer, aresemiconductors with a tiny bandgap; and all others are semiconductorswith a bandgap that inversely depends on the nanotube diameter. Sincemethods are available for selecting carbon nanotubes according toconductivity type, the choice of nanotubes for particular applicationscan be optimized. The optimal SWNT type depends upon the device ormaterials application. Metallic nanotubes are generally suitable forapplications where maximum conductivity is desired, such as fortransparent conductors or as counter electrodes where rapid responserate is desired. On the other hand, semiconducting nanotubes are desiredfor the channel material for semiconductor devices of the presentinvention.

Various methods of separating single wall and multiwall nanotubes (bytype, length, diameter, etc.) are useful for invention embodiments.Examples of known methods for such separation involve (1) use of chargetransfer agents that complex most readily with metallic nanotubes, (2)complexation with selected types of DNA, and (3) dielectrophoresis (R.Krupke et al., Nano Letters 3, 1019-1023 (2003) and R. C. Haddon et al.,MRS Bulletin 29, 252-259 (2004)). These and other methods can be used toprovide nanotube materials for the practice of this invention, such asthe nanofiber transistors of FIGS. 11 and 12. The variousdielectrophoretic methods are especially useful for depositing nanotubesof a desired conducting type (metallic (or small bandgap) orsemiconducting nanotubes) on a device substrate between two electrodes(which are used to apply the alternating current potential used for thedielectrophoresis). Depending upon the frequency of the voltage appliedfor dielectrophoretic deposition, either metallic (and near metallicnanotube having very small bandgaps) or semiconducting nanotubes (havingmuch larger band gaps) can be preferentially deposited on semiconductorsubstrates. If the nanotubes in solution are partially bundled, and thechosen frequency for the dielectrophoretic process favors deposition ofmetallic nanotubes, the deposited nanotubes can be bundles containing ametallic nanotube (or very small bandgap semiconducting nanotube)together with semiconducting nanotubes. More generally, if it isdesirable to remove metallic nanotubes from a single walled nanotubebundle (or remove a metallic nanotube from being the outermost wall on amultiwall nanotube, well-known high voltage pulse methods can be used(see P. Collins et al., Science 292, 706-709, (2001) and A. Javey etal., Nano Letters 2, 929-932 (2002)).

Synthetic methods generally result in mixtures of nanotubes havingdifferent diameters. Use of catalyst for nanotube synthesis that isclose to monodispersed in size (and stable in size at the temperaturesused for synthesis) can dramatically decrease the polydispersity innanotube diameters, and having this narrower range of nanotube diameterscan be useful for invention embodiments. S. M. Bachilo et al. describesuch a method in Journal of the American Chemical Society 125,11186-11187 (2003).

Nanotubes and nanoscrolls filled with agents that provide chargetransfer to the nanotubes are included in preferred electrodecompositions, including those used as device channels. One reason forthis preference is that these agents effectively bias the degree ofcharge transfer to the nanotubes and nanoscrolls, thereby shifting theFermi level of the nanotubes and consequently the density of states atthe Fermi level. The filled volume of the nanotubes should have a netnon-zero charge and the counter charge to this net non-zero change canpredominately reside on or near the external surface of the nanotube ornanoscroll. Examples of filling agents that can have a net non-zerocharge when inside the nanotubes or nanoscrolls are ionic salts and saltsolutions, partially ionized elements (such as alkali metals anddivalent elements such as calcium), and organic transfer agents. For thepurposes of this invention embodiment, ionic salts and ionic saltsolutions inside nanotubes and nanoscrolls are not considered to becontacting electrolytes. Other suitable filling agents for nanotubes andnanoscrolls are fullenenes (especially C₆₀) and fullerenes complexedwith charge transfer agents (like monovalent, divalent, and trivalentelemental metals). The complexed fullerenes that are included within thevolume of the nanotube or nanoscroll can optionally comprise endohedralfullerenes that contain n species Z that are within the C_(m), which aredesignated using the symbols Z_(n)@C_(m). Typical examples are Gd@C₈₂,La@C₈₂, La₂@C₈₂, Sm@C₈₂, Dy@C₈₂, Ti₂C₈₀, La₂C₈₀, and like compositions.

Various methods are particularly useful in invention embodiments forfilling or partially filling nanotubes. These methods typically includea first step of opening nanotube ends, which is convenientlyaccomplished using gas phase oxidants, other oxidants (like oxidizingacids), or mechanical cutting. The opened nanotubes can be filled invarious ways, like vapor, liquid phase, melt phase, or supercriticalphase transport into the nanotube. Methods for filling nanotubes withmetal oxides, metal halides, and related materials can be like thoseused in the prior art to fill nanotubes with mixtures of KCl and UCl₄;KI; mixtures of AgCl and either AgBr or Agl; CdCl₂; Cdl₂; ThCl₄; LnCl₃;ZrCl₃; ZrCl₄, MoCl₃, FeCl₃, and Sb₂O₃. In an optional additional step,the thereby filled (or partially filled) nanotubes can be optionallytreated to transform the material inside the nanotube, such as bychemical reduction or thermal pyrolysis of a metal salt to produce ametal, such as Ru, Bi, Au, Pt, Pd, and Ag. M. Monthioux, Carbon 40,1809-1823 (2002) provides a useful review of these methods for fillingand partially filling nanotubes, including the filling of nanotubesduring nanotube synthesis. The partial or complete filling of variousother materials useful for invention embodiments is described in J.Sloan et al., J. Materials Chemistry 7, 1089-1095 (1997).

Conducting nanofibers need not contain carbon in order to be useful forinvention embodiments, and a host of processes are well known in the artfor making suitable nanofibers. Some examples are the growth ofsuperconducting MgB₂ nanowires by the reaction of single crystal Bnanowires with the vapor of Mg (Y. Wu et al., Advanced Materials 13,1487 (2001)), the growth of superconducting lead nanowires by thethermal decomposition of lead acetate in ethylene glycol (Y. Wu et al.,Nano Letters 3, 1163-1166 (2003)), the solution phase growth of seleniumnanowires from colloidal particles (B. Gates et al., J. Am. Chem. Soc.122, 12582-12583 (2000) and B. T. Mayer et al., Chemistry of Materials15, 3852-3858 (2003)), and the synthesis of lead nanowires by templatinglead within channels in porous membranes or steps on silicon substrates.The latter methods and various other methods of producing metal andsemiconducting nanowires suitable for the practice of inventionembodiments are described in Wu et al., Nano Letters 3, 1163-1166(2003), and are elaborated in associated references. Y. Li et al. (J.Am. Chem. Soc. 123, 9904-9905 (2001)) has shown how to make bismuthnanotubes. Also, X. Duan and C. M. Lieber (Advanced Materials 12,298-302 (2000)) have shown that bulk quantities of semiconductornanofibers having high purity can be made using laser-assisted catalyticgrowth. These obtained nanofibers are especially useful for inventionembodiments and include single crystal nanofibers of binary group III-Vmaterials (GaAs, GaP, InAs, InP), tertiary III-V materials (GaAs/P,InAs/P), binary II-VI compounds (ZnS, ZnSe, CdS, and CdSe), and binarySiGe alloys. Si nanofibers, and doped Si nanofibers, are also useful forinvention embodiments. The preparation of Si nanofibers by laserablation is described by B. Li et al. (Phys. Rev. B 59, 1645-1648(1999)). Various methods for making nanotubes of a host of usefulmaterials are described by R. Tenne in Angew. Chem. Int. Ed. 42,5124-5132 (2003). Also, nanotubes of GaN can be usefully made byexitaxial growth of thin GaN layers on ZnO nanowires, followed by theremoval of the ZnO (see J. Goldberger et al., Nature 422, 599-602(2003)). Nanofibers having approximate composition MoS_(9−x)I_(x), whichare commercially available from Mo6 (Teslova 30, 1000 Ljubljana,Slovenia) are included as useful compositions (most especially for xbetween about 4.5 and 6). Related compositions are also described by D.Vrbanić et al. in Nanotechnology 15, 635-638 (2004). Among otherapplications embodiments, these latter fibers are useful for electronemission tips and as a tunable superconductor.

Another way to make the high-surface-area materials used in inventionembodiments is by templating a self-assembled structure that has highsurface area. For example, a periodic template crystal (called an opal)can be obtained by the sedimentation of spheres that are substantiallymonodispersed in diameter. These spheres typically have an averagesphere diameter of between 500 nm and 10 nm. In most cases, thesespheres are from either (a) an inorganic oxide, such as SiO₂, which canbe removed by chemical processes such as exposure to aqueous acid orbase, or (b) an organic polymer that can be removed by pyrolysis,chemical reaction, or dissolution. The template crystal, after anoptional sintering process to provide inter-sphere necks, is infiltratedwith either the electrode material or a material that can be convertedto the electrode material. This sintering process in described byZakhidov et al. in Science 282, 897 (1998) and U.S. Pat. No. 6,261,469and U.S. Pat. No. 6,517,762. Thereafter, the template material isremoved to provide an inverse lattice, which is a structural replica ofthe original template crystal. As an example, Zakhidov et al. (Science282, 897 (1998)) used plasma-enhanced CVD to make a very high surfacearea graphitic carbon. Millimeter thick opal plates based onmonodispersed SiO₂ spheres were infiltrated with carbon from ahydrogen/methane plasma created by microwave excitation. Extraction ofthe SiO₂ spheres from the carbon infiltrated opal (using aqueous HF)resulted in a very high surface area, nanoporous foam in which carbonlayers as thin as 40 Å make the internal surface of the foam. As anothersuitable method for fabrication conducting sphere arrays having highsurface area for use as nanostructured electrodes, conducting spheresthat are nearly monodispersed in diameter can be directly assembled fromsphere dispersions using conventional methods. These spheres aretypically less than about 200 nm in average diameter, more typicallyless than 100 nm in average sphere diameter, and most typically lessthan 50 nm in average sphere diameter.

The devices of this invention may comprise more that twocurrent-carrying electrodes that can be operated at different voltages.Advantages of using more than two current carrying electrodes are thatadditional flexibility is achieved with respect to the degree of chargeinjection in the individual electrodes, which can be useful foroptimizing device performance.

In some processes of the present invention, electrode charging for atleast one electrode is predominately non-faradaic, meaning that over 50%of the initially injected charge is injected non-faradaically. It shouldbe emphasized that this definition pertains to the nature of initialcharge injection from the electrolyte, since initially non-faradaicallyinjected charge can later transform to charge that is retainedfaradaically during electrode drying processes. Such is the case ifdopant ions that are initially stored in an electrochemical double layerlater intercalate into the electrode material. In the non-faradaicprocess of certain embodiments, the ions from the electrolyte (whichcompensate the electronic charges injected into the electrodes) arelocated close to the surface of at least one of these electrodeelements. This is in contrast to the faradaic processes where the ionsthat compensate the injected electronic charges penetrate inside theelectrode material and change its structure, usually expanding it. Insome invention embodiments, typically over 50% of the stored charge isstored non-faradaically in the charged material, meaning that at least50% of this stored charge is associated with ions on or near the surfaceof the material. In some invention embodiments, typically over 75% ofthe stored charge is stored non-faradaically in the electronicallycharged material, meaning that at least 75% of this stored charge isassociated with ions on or near the surface of the material. Because ofthis location of charge on material surfaces in the non-faradaicprocesses of invention embodiments, ions from the electrolyte need notpenetrate the electrode material and need not cause phase changes withinthe electrode material. This use of non-faradaic charge injection isgenerally most important for device applications where the chargeinjected electrode material must be repeatedly charged and dischargedduring device operation, since non-faradaic charge injection generallyprovides longer device cycle life than does faradaic charge injection.It should be recognized that charge injection processes can benon-faradaic over one potential range, and then become faradaic whenthis potential range is extended. Consequently the definitions ofpredominately non-faradaic and predominately non-faradaic indirectlysignify the potential range used for hole or electron charging. A deviceis called a predominately non-faradaic as long as there is a deviceoperation range where a useful device response can be obtained frompredominately non-faradaic charging of at least one electrode. Whilecharge that is initially inserted predominately non-faradaically canlater produce materials intercalation (i.e., ions insertion into solidmaterial volume), it is advantageous if less than 50% of counter ioncharge associated with maximum injected electronic charge isintercalated in a solid material during material use or normal deviceoperation.

The achievement of very high electrode capacitances requires the use ofnanostructured materials that have small sizes in at least onedimension, and such small dimensions can affect the properties of thenanostructured material in both charge-injected and non-charge-injectedstates. Consequently, the use of materials with larger dimensions (sheetthicknesses, fiber diameters, or particle sizes) can in some cases besuitable (with a corresponding decrease in suitable electrodecapacitances)—especially for the tuning of highly-scale-sensitiveproperties, such as ferromagnetism.

The case where only the working electrode operates predominatelynon-faradaically is also included in invention embodiments. Devices inwhich ions predominately insert on the internal and external surface ofone electrode and in the material volume of the second electrode areincluded here. This can be a useful embodiment in cases where cycle lifelimitations are not problematic for the desired application mode orwhere the potentially higher charge storage densities of faradaicprocesses provide needed benefits of reduced device size or weight.However, predominately non-faradaic operation of both electrodes isuseful when very long device cycle lifetimes are needed, whencharge-injection modified properties of both electrodes are utilized, orwhen dopant intercalation and associated structural changes degradeneeded properties.

The device types of some embodiments have at least one electrode(typically the working electrode) that has high gravimetric surfacearea, since this high surface area is typically required to obtain highdegrees of double-layer (non-faradaic) charge injection. In fact,material selection to provide either predominately non-faradaic orpredominately faradaic performance is made according to either surfacearea measurements or structural results (from typically scanningelectron microscopy, SEM, electron transmission microscopy, TEM, oratomic probe microscopies). The gravimetric surface area is convenientlytaken as the surface area measured in nitrogen gas by the standardBrunauer-Emmett-Teller (BET) method. The gravimetric surface area of theworking electrode is advantageously above about 1 m²/g, moreadvantageously above about 10 m²/g, and most advantageously above about100 m²/g. In some instances where the electrodes must be repeatedlycharged and discharged during device operation and long cycle life isneeded, the surface area of both working and counter electrodes isadvantageously above 1 m²/g, more advantageously above about 10 m²/g,and most advantageously above about 100 m²/g.

It is advantageous for some device embodiments that either the workingelectrode or both working and counter electrodes comprise a mixture ofelectronically conductive materials that serve as electrode components.This mixture of materials in the electrodes can include materials thatare non-faradaically charged and faradaically charged.

The working and counter-electrodes can be made of either the same ordifferent materials. Suitable examples of electrode materials include(a) high surface area metallic compositions obtained by the degeneratedoping of semiconductors (such as Si, Ge, n-doped or p-doped cubic boronnitride, mixtures of Si and Ge, and GaAs), (b) conducting forms ofconjugated organic polymers (such as polyacetylene, poly(p-phenylene),or poly(p-phenylene vinylene) and copolymers thereof), (c) carbonaceousmaterials obtained by the pyrolysis and surface area enhancement ofpolymers, (d) graphite, carbon nanotubes, and less ordered forms ofcarbon formed by pyrolysis, (e) elemental metals and alloys of thesemetals, and (f) electrically conducting metal oxides and metalchalcogenides (such as CdS and CdSe). Doped diamond is another suitableelectrode material, especially hole-doped diamond (which is a conductorand even a superconductor—see E. A. Ekimov et al., Nature 428, 542-545(2004)). Use of this degeneratively doped diamond as a predominatelynon-faradaically charge injected material, this diamond should beconfigured as a high surface area material. Methods for makingnanoporous diamond having high surface area are described by A. A.Zakhidov et al. in Science 282, 897 (1998), U.S. Pat. No. 6,261,469, andU.S. Pat. No. 6,517,762.

Organic conducting polymers are among the suitable compositions for useas predominately faradaic electrodes. Very high surface area conductingpolymers are included as suitable compositions for predominatelynon-faradaic electrodes. Various methods can be used to obtainconducting polymer electrodes having high surface areas. For example,known methods can be used for the electrostatic spinning of conductingpolymers into nanometer diameter fibers. For these conducting polymersthe predominately non-faradaic behavior is obtained as a result of thishigh surface area and operation of the electrodes in potential rangeswhere the major faradaic process do not substantially occur. Especiallysuitable organic conducting polymers are those with planar or nearlyplanar backbones, such as poly(p-phenylene), poly(p-phenylene vinylene),and polyacetylene. Other suitable conducting polymers are variousconducting polypyrroles, polyanilines, polyalkyithiophenes, andpolyarylvinylenes. The synthesis of conducting polymers suitable forsuch embodiments is well known, and is described, for example, in theHandbook of Conducting Polymers, Second Edition, Eds. T. A. Skotheim etal. (Marcel Dekker, New York, 1998).

The nanostructured conductor (such as a nanotube) used as a chargeinjected electrode can optionally be coated with another material. Iffact, benefits can result even if this over-coated material is a poorelectronic conductor. However, if the over-coated material is a poorconductor, provision should be made to insure that charge injection intothe nanostructured material can occur. This is accomplished, forexample, by either making direct electrical contact to thenanostructured conductor or insuring that the over-coated poorlyconducting material is sufficiently thin that tunneling or otherelectronic transport is possible (on the desired time scale) across thispoorly conducting material.

Various benefits can result for over-coating the nanostructuredelectrode material with a second material. As a first benefit, thisover-coated material can serve to protect the charge injected state ofthis conducting nanostructured material against undesired reaction withredox-active impurities in the environment. This can be the case, forexample, when electronic charge injection into a nanostructured materialis enabled by ion motion internal to this nanostructured material, suchas on the inside of a nanotube or in one of the two possiblenon-interconnected labyrinths of an inverse lattice photonic crystal. Asecond achievable benefit is most effectively realized when the ions forthe electronically injected charge are on the exterior surface of theover-coated material, so that this over coated material is between theions and the nanostructured base material. This second benefit is thatthe enormous electric field generated by charge injection is appliedacross the over coated material, which can provide useful electronic,magnetic, or optical properties for this material by causing some of thecharge to be injected into the over-coated material.

A host of methods can be used for providing this over-coating layer,such a vapor state, liquid state, super-fluid-state coating of thenanostructured electrode material. Also the nanostructured material canbe formed inside an insulating over coating material after this overcoating material if formed, such as by the filling of insulating boronnitride nanotubes with C₆₀ and the subsequent coalescence of arrays ofthese C₆₀ molecules to form carbon nanotubes as the conductingnanostructured electrode material. (See W. Mickelson et al., Science300, 467-469 (2003) for this process for forming carbon nanotubes insideBN nanotubes.)

Aerogels, and especially carbon aerogels and aerogels based onconducting organic polymers, are included in the list of suitableelectrode materials. Resorcinal-formaldehyde-derived carbon aerogels areespecially suitable carbon aerogels. These carbon aerogels can beconveniently produced using the sol gel method, supercritical dryingusing liquid CO₂, and pyrolysis in nitrogen at about 1000° C. Thesynthesis of these carbon aerogels is described by Salinger et al. inJournal of Non-Crystalline Solids 225, 81 (1998) and by Wang et al., inJournal of the Electrochemical Society 148, D75-D77 (2001). Otheraerogels that are useful for invention embodiments are described by J.L. Mohanan et al. in Science 307, 397-399 (2005).

Various methods are well known in the literature for assemblingnanostructured fibers and particles into forms well suited for thepractice of the present invention. For example, sheet shaped electrodesof single-wall nanotubes can be conveniently formed by filtering anaqueous suspension of such purified carbon tubes throughpoly(tetrafluoroethylene) filter paper, as described by Lui et al. inScience 280, 1253 (1998). Peeling the resulting paper-like sheet fromthe filter results in a free standing sheet of carbon nanotube bundles.This sheet, which can conveniently range in thickness from 0.1-100microns, possesses mechanical strength, which is derived from themicro-scale entanglement of the nanotube bundles. In order to increasethe mechanical properties of these sheets for the applications, it isadvantageous for the nanotube sheets to be annealed at a temperature ofat least 400° C. for 0.5 hour or longer prior to use. Moreadvantageously, these nanotube sheets should be annealed at atemperature of at least 1000° C. for 0.5 hour or longer in either aninert atmosphere or a hydrogen-containing atmosphere. In order topreserve the nanotube structure, this anneal temperature is preferablybelow 2000° C. Alternatively, carbon nanotubes can be deposited on asurface by deposition from a dispersion of nanotubes in a liquid, suchas dichloroethane or water, which contains a surfactant (such as TritonX-100 from Aldrich, Milwaukee, Wis.).

The relative and absolute sizes of working and counter-electrodes areimportant for determining optimal device design. Consider first the casewhere both working and counter electrodes operate predominatelynon-faradaically. In order to obtain rapid switching of the propertiesof the working electrode (without the necessity of faradaic processes inthe electrolyte that can degrade cycle life), the total surface area ofthe counter electrode should generally be at least about twice as largeas the working electrode. If a more rapid device response is required,the total surface area of the counter electrode can be at least aboutten times larger than that of the working electrode. For microdeviceshaving extremely fast switching rates, it is most advantageous that thetotal surface area of the counter electrode is at least about a hundredtimes larger than that of the working electrode. The conditions for highrate switching of the properties of the properties of the workingelectrode can also be expressed in terms of electrode capacitance, wherethe ratio of counter-electrode to working-electrode capacitances isadvantageously at least about 2, more advantageously at least about tentimes, and most advantageously (for certain high rate devices) about atleast a hundred times larger than that of the working electrode.

In addition to optimizing rate performance, these relative surface areasand relative electrode capacitances determine the fraction of totalapplied inter-electrode potential that is applied across each individualelectrode. Since the total potential that can be applied is typicallydetermined by the stability of the electrolyte, the selection ofincreasingly large values for these ratios is also desirable forincreasing the fraction of the applied potential that is applied acrossthe working electrode, and therefore increasing the amount of chargeinjection in this electrode and the corresponding degree of propertieschange for this electrode. On the other hand, the selection of relativeworking and counter-electrode capacitances and surface areas close tounity can be desirable if substantial properties switching is needed forboth electrodes or if there is a need to minimize device size or weight.Device response rate decreases with increasing size and increasingthickness electrodes, and the lowest capacitance electrode typicallydetermines response rate. Hence, for applications were high charge anddischarge rates are needed, the thickness of the lowest capacityelectrode should be advantageously below about five millimeters (5000microns), more advantageously below about 1000 microns, even moreadvantageously below about 100 microns, and most advantageously below 50microns. However, it should be understood from the present teachingsthat much thinner electrodes can be desirably used when very fast rateresponse is not a performance issue and much thinner electrodes can beused in microscopic devices where very small sizes and very highcharge/discharge rates are required. Charge injected materials that arelarger than a micron in the smallest external dimension are useful forapplications where the substantial material volumes are required. Inorder to achieve large degrees of charge injection, such materialsshould ideally contain at least about 50% void volume.

With further regard to the rate of device response, the rate response(as a fraction of the maximum achievable response) increases withincreasing values of (R_(S)C_(S))⁻¹, where R_(S) is the effectiveresistance of the electrochemical system and C_(S) is the effectivecapacitance of this system. Key contributions to R_(S) can come from theresistivities of the working and counter electrodes and the resistivityof the electrolyte for ionic conduction. Consequently, it isadvantageous that the device contains at least one electrode having anelectronic conductivity at room temperature that is above 1 S/cm. Moreadvantageously at least one electrode has an electronic conductivity atroom temperature that is above 100 S/cm. Even more advantageously atleast one electrode has an electronic conductivity at room temperaturethat is above 1000 S/cm. Most advantageously, the device contains atleast two electrodes that each has an electronic conductivity at roomtemperature that is above 1000 S/cm. In general, the electricalconductivities of nanotube assemblies will be anisotropic and dependupon the degree of charge injection. In such cases, the conductivitiesreferred to above correspond to the highest conductivity direction ofthe conducting material in the most conducting state obtained by chargeinjection.

From a viewpoint of having fast charge and discharge rates, the ionicconductivity of the electrolyte is advantageously above 10⁻⁴ S/cm, moreadvantageously above about 10⁻³ S/cm, and most advantageously aboveabout 10⁻¹ S/cm. In addition, device response rates are enhanced for themore poorly conducting electrolytes by minimizing the average thicknessof electrolyte that separates the at least two typically required deviceelectrodes. The maximum electrolyte thickness that separates the twotypically required electrodes is preferably less than 1 millimeter whenthese electrolytes are solid-state inorganic or organic electrolytes.More preferably this average electrolyte thickness is less than 0.1millimeters for solid-state electrolytes. However, for highly conductingionic fluids (such as aqueous salts like aqueous NaCl, aqueous baseslike aqueous KOH, and aqueous acids like aqueous sulfuric acid) muchlarger inter-electrode separations can be used without adverselyeffecting charge and discharge rates.

Various inorganic and organic liquid, gel, and solid electrolytes can beused for preferred invention embodiments. Generally, liquid electrolytesare ideally suitable for the processes of this invention in which chargeis electrochemically induced non-faradaically in a material, and thecharge and associated properties changes are retained when the electrodematerial is disconnected from the power source and the electrolyte isremoved from the electrode material. The reason for this suitability isthat the convenient means are available for removal of liquidelectrolytes from the charge-injected material without eliminating thecharge injection (such as simple evaporation of the solvent component ofthe electrolyte). On the other hand, solid-state electrolytes aresuitable for devices that retain the used electrolyte, since the use ofsolid-state electrolytes eliminates the problems of liquid electrolytecontainment and incompatibility with the generic strategiesconventionally used for device fabrication. There are tradeoffs betweenthese different electrolyte types with respect to the allowabletemperature and voltage operating ranges and the obtainable electricalconductivities. An electrolyte that is suitable (because of its low costand high ionic conductivity) is water containing simple salts, like 1 MNaCl or 1 M KCl. Other very high ionic conductivity electrolytes (likeconcentrated aqueous KOH and sulfuric acid) are also suitable forproviding rapid charging and discharge. Aqueous electrolytes comprisingat least about 4 M aqueous H₂SO₄, or 4 M aqueous KOH, are especiallysuitable for application embodiments where the electrolyte is used onlyfor materials processing by charge injection. Aqueous electrolytescomprising about 38 weight percent H₂SO₄ and electrolytes comprisingabove 5 M aqueous KOH are most especially suitable. For applicationswhere a large degree of charge injection is needed, electrolytes withlarge redox windows are suitable. Especially suitable organicelectrolytes include propylene carbonate, ethylene carbonate, butylenecarbonate, diethyl carbonate, dimethylene carbonate, and mixturesthereof with salts such as LiClO₄, LiAsF₆, LiPF₆, LiBF₄, LiCF₃SO₃,Li(CF₃SO₂)₂N, and Li(CF₃SO₂)₃C. Ionic liquid electrolytes (like1-butyl-3-methyl imidazolium tetrafluoroborate) and ionic liquids inpolymer matrices are especially suitable because of the achievable wideredox stability range and the cycle life that they provide for redoxcycling

Solid-state electrolytes can also be used advantageously, since suchelectrolytes enable all-solid-state devices. Suitable organic-basedsolid-state electrolytes are polyacrylonitrile-based solid polymerelectrolytes (with salts such as potassium, lithium, magnesium, orcopper perchlorate, LiAsF₆, and LiN(CF₃SO₂)₂). Suitable organic solventsfor these solid-state and gel electrolytes include propylene carbonate,ethylene carbonate, γ-butyrolactone, and mixtures thereof. Suitable gelor elastomeric solid electrolytes include lithium salt-containingcopolymers of polyethylene oxide (because of high redox stabilitywindows, high electrical conductivities, and achievable elastomericproperties), electrolytes based on the random copolymerpoly(epichloridrin-co-ethylene oxide), phosphoric acid containing nylons(such as nylon 6,10 or nylon 6), and hydrated poly(vinyl alcohol)/H₃PO₄.Other suitable gel electrolytes include polyethylene oxide andpolyacrylonitrile-based electrolytes with lithium salts (like LiClO₄)and ethylene and propylene carbonate plasticizers. The so called“polymer in salt” elastomers (S. S. Zhang and C. A. Angell, J.Electrochem. Soc. 143, 4047 (1996)) are also suitable forlithium-ion-based devices, since they provide very high lithium ionconductivities, elastomeric properties, and a wide redox stabilitywindow (4.5-5.5 V versus Li⁺/Li). Suitable electrolytes for hightemperature device applications include ionic glasses based on lithiumion conducting ceramics (superionic glasses), ion exchanged β-alumina(up to 1,000° C.), CaF₂, La₂Mo₂O₉ (above about 580° C.) and ZrO₂/Y₂O₃(up to 2,000° C.). Other suitable inorganic solid-state electrolytes areAgI, AgBr, and Ag₄RbI₅. Suitable inorganic molten salt electrolytes forhigh temperature devices include alkali metal halides (such as NaCl,KCl, and mixtures of these salts) and divalent metal halides (such asPbCl₂). Some of the proton-conducting electrolytes that are useful ininvention embodiments as the solid-state electrolyte include, amongother possibilities, Nafion, S-PEEK-1.6 (a sulfonated polyether etherketone), S-PBI (a sulfonated polybenzimidazole), and phosphoric acidcomplexes of nylon, polyvinyl alcohol, polyacryamide, and apolybenzimidazole (such aspoly[2,2′-(m-phenylene)-5,5′-bibenzimidazole].

The devices of some embodiments can use either one electrolyte or morethan one electrolyte. For example, the electrolyte that contacts part ofa porous nanostructured electrode can be different from the electrolytethat further provides an ion conducting path between electrodes. Also,different electrolytes can be used as contacting materials for differentelectrodes. Employing more than one electrolyte can be used to optimizedevice operation. For instance, a particular electrolyte (orelectrolytes) can be chosen for optimizing either double-layer formationor electrode ionic conductivity. While the electrolyte that separateselectrodes is substantially electronically insulating, the ion conductorthat contacts an individual electrode can have a significant degree ofelectronic conductivity. In fact, conducting polymers are used as theelectrode conducting element for certain described inventionembodiments. Like electrolytes, these conducting polymers can provideion conduction and serve as an ion source. However, unlike electrolytes,these conducting polymers are electronically conducting. Hence, it isadvantageous that these conducting polymers do not provide anuninterrupted electronic path between opposite electrodes. For thisreason, conducting polymers (or other ion-intercalated electronicconductors) are advantageously used in combination with one or moreelectronically insulating electrolyte to form a inter-electrode pathwaythat is largely interrupted for inter-electrode electronic transport,but maintained for inter-electrode ion transport.

The following examples are provided to demonstrate particularembodiments of the present invention. It should be appreciated by thoseof skill in the art that the methods disclosed in the examples whichfollow merely represent exemplary embodiments of the present invention.However, those of skill in the art should, in light of the presentdisclosure, appreciate that many changes can be made in the specificembodiments described and still obtain a like or similar result withoutdeparting from the spirit and scope of the present invention.

Example 1

This example shows that the electrical conductivity of carbon nanotubeelectrode sheets can be continuously varied by about an order ofmagnitude using predominately non-faradaic electrochemical chargeinjection in 1 M NaCl electrolyte

These nanotube sheets are fabricated analogously to a previouslydescribed method (A. G. Rinzler et al., Appl. Phys. A 67, 29 (1998)) bydispersing the carbon nanotubes in surfactant-containing aqueoussolution and filtering this nanotube dispersion though through a 47 mmdiameter poly(tetrafluoroethylene) filter sheet (Millipore LS) underhouse vacuum. A sheet of nanotubes collected on the filter paper, whichwas washed using water and methanol, dried, and then lifted from thefilter paper substrate to provide a free-standing SWNT sheet. Thedensity of these nanotube sheets is about 0.3 g/cm³, versus the densityof about 1.3 g/cm³ for densely packed nanotubes having close to theobserved average nanotube diameter. Hence the void volume in thesenanotube sheets is about 76.9 volume percent. This high void volume, andthe correspondingly high accessible surface area, is generally importantfor achieving high degrees of non-faradaic charge injection. Supportingthis conclusion, the measured BET surface area determined from nitrogengas adsorption for these nanotube sheets is approximately 300 m²/g.

Two carbon nanotube electrodes (one small: ˜10 mm wide and ˜30 mm long,and the other large: ˜1 mm wide and ˜30 mm long) were cut fromfree-standing carbon nanotube sheets made by the above described method.For the purpose of making four-probe electrical conductivitymeasurement, four in-series electrical contacts were made to the smallcarbon nanotube electrode. Gold wire (0.0127 mm diameter) was attachedusing heat cured conductive epoxy (H20E, Epoxy Technology), which wascured at 70° C. for four hours. The area of electrical contact wascovered by chemically resistive epoxy (Eccobond A 316, Emerson & Cuming)in order to protect the electrical contact from exposure to subsequentlyused electrolytes. One electrical contact was similarly made to thelarge carbon electrode.

The small carbon nanotube electrode with four-probe contacts, the largecarbon nanotube electrode having one electrical contact, and the Ag/AgClreference electrode were immersed in 1M aqueous NaCl. This selectedconfiguration (with the larger electrode about ten times smaller thanthe larger electrode) keeps the potential of the bigger electrode almostconstant as a potential difference is applied between the smallelectrode (working electrode) and the large electrode (workingelectrode), to thereby change the potential of the of the smallelectrode relative to reference electrode. The four-point electricalconductivity of the small electrode was measured as a function of thepotential of this electrode with respect to the Ag/AgCl referenceelectrode.

Multiple experiments using the above-described method provided thefollowing results. When the potential of the carbon nanotube paperelectrode (versus Ag/AgCl) was in the range of about −0.35 to about−0.50V, the conductivity showed the minimum of 60-100 S/cm. Thisconductivity for an uncharged carbon nanotube sheet monotonicallyincreased up to about 1000 S/cm when the applied potential was increasedto above +0.9V, and it monotonically increased up to about 250 S/cm whenthe potential was decreased to below −0.9V. The dependence ofconductivity change upon potential (versus Ag/AgCl) was quite symmetricwith respect to the potential at which the conductivity minimum occurs(at between about −0.35 about −0.50V).

Cyclic voltammetry in this potential range shows that there are nonoticeable peaks due to Faradaic processes in the utilized potentialrange, indicating that the major electrical conductivity increases (upto an order of magnitude) are the result of predominately non-faradaiccharging.

Example 2

This example demonstrates that tuning the electrical conductivity ofsingle-wall nanotube sheets over an order-of-magnitude range can also beaccomplished using an organic electrolyte, instead of the aqueouselectrolyte of Example 1.

The experimental procedure was exactly the same as for Example 1, exceptthat the 1M NaCl aqueous electrolyte was replaced with either 0.1MTBAPF₆ (tetrabutyl ammonium hexafluorophosphate) or 0.1M TBABF₄(tetrabutyl ammonium tetrafluoroborate) dissolved in acetonitrile andthe Ag/AgCl reference electrode was replaced with the Ag/Ag⁺ referenceelectrode. There were no significant differences noted betweencharge-injection tuning of the nanotube sheet conductivity in 0.1MTBAPF₆/acetonitrile and in 0.1 M TBABF₄/acetonitrile.

Because the potential window of non-faradaic reaction in non-aqueouselectrolyte is much wider than for aqueous electrolytes, the potentialcan be changed up to +1.0V (versus Ag/Ag⁺) in the positive potentialdirection, resulting in a maximum electrical conductivity of above 1000S/cm at these positive potentials. In the negative potential direction,non-faradaic charging was possible down to −1.5V (versus Ag/Ag⁺),resulting in a maximum electrical conductivity of about 600-700 S/cm atthese negative potentials (where electrons are being injected into thenanotubes). The minimum value was ˜100 S/cm near ˜0.3V (versus Ag/Ag⁺).

FIG. 1 shows the presently observed continuous tunability of four-pointelectrical conductivity as a function of applied potential (versusAg/Ag+) for a sheet of single-wall carbon nanotubes immersed in one ofthese organic electrolytes (0.1 M tetrabutylammonium hexafluorophosphatein acetonitrile). These results show that the electrical conductivitycan be increased by over an order of magnitude by electrochemical chargeinjection. There is slight hysteresis evident for the curves in FIG. 1,with the conductivity a on the extreme left side of the potentialminimum slightly higher for hole injection (increasingly positiveapplied potential) and the conductivity slightly lower on the extremeright side of the minimum for electron injection (increasingly negativeapplied potentials). The different curves are for three successivecycles (using squares (101), circles (102), and triangles (103) forsuccessive cycles).

FIG. 3 shows measured cyclic voltammetry during charge injection for theabove SWNT sheet when immersed in the above tetrabutylammoniumhexafluorophosphate electrolyte. The absence of major peaks in thiscyclic voltammetry curve (measured versus Ag/Ag+) indicates thatcharging is predominately non-faradaic for this electrolyte andpotential range.

Example 3

This example shows that the hysteresis in properties tuning can besignificantly reduced if electrical conductivity is varied by chargingthe amount of injected charge, as opposed to being controlled bychanging applied voltage. This point is illustrated for the 0.1Mtetrabutylammonium hexafluorophosphate/acetonitrile electrolyte bycomparing the hysteresis in the electrical conductivity versus potential(FIG. 1 and Example 1) with those in FIG. 2, where electricalconductivity is plotted versus charge per carbon in the nanotube workingelectrode. The decrease that is obtained in hysteresis in going fromvoltage control of conductivity to charge control is even larger for theaqueous 1 M NaCl electrolyte of Example 1. The data in FIG. 2 also showsthe dependence of four-point electrical conductivity upon the amount ofinjected charge (per carbon) for the nanotube sheet is nearly identicalfor the 1 M NaCl electrolyte of Example 1 and the 0.1Mtetrabutylammonium hexafluorophosphate/acetonitrile electrolyte ofExample 2. For the results in FIG. 2, the black data points are forexperiments using 0.1M tetrabutylammoniumhexafluorophosphate/acetonitrile electrolyte and near-white data pointsare for measurements using 1 M aqueous NaCl electrolyte. The origin ofthe charge/carbon scale in FIG. 2 is arbitrary.

Example 4

The unexpected results described in this example show that chargenon-faradaically injected into carbon nanotube sheets is retained to alarge extent when the carbon nanotube sheet is disconnected from thepower source, and then dried in either air or in flowing nitrogen gas.This retention of injected charge is indicated by substantial retentionof the electrical conductivity enhancement caused by charge injection.

The nanotube sheet preparation and the method of charge injection andconductivity measurement is the same as in the above examples. Theelectrolyte used is the 0.1M tetrabutylammoniumhexafluorophosphate/acetonitrile of Examples 2 and 3.

FIG. 4 shows that the dramatic hole-injection-induced increase inelectrical conductivity of the nanotube sheet in FIGS. 1 and 2 islargely retained when the hole-injected electrode is dried in flowingdry nitrogen atmosphere to remove the electrolyte. The insert to thisfigure shows results over a four-hour period on an expanded time scale.FIG. 5 shows the retention of conductivity enhancement when the nanotubesheet is removed from the electrolyte and held in air for theinvestigated five day period.

Example 5

This example demonstrates the generality of non-faradaically injectingcharge in a nanostructured material in accordance with embodiments ofthe present invention, and maintaining this injected charge andassociated properties changes when the charge injected material isremoved from the electrolyte and dried. More specifically, it is shownthat charge non-faradaically injected into platinum nanoparticle pelletsis substantially maintained even after disconnecting the platinumpellets from the power source, their removal from the electrolyte, andexposure of these pellets to a dynamically pumped vacuum for a week.

The platinum nanoparticle pellets investigated here were made at roomtemperature by compaction of 30 nm platinum nanoparticles. This methodused for making the investigated pellets is like that described by J.Weissmüller et al. in Science 300, 312 (2003). The method used forattaching electrodes to the platinum pellets is the same as thatdescribed in Example 1 for the carbon nanotube sheets.

The density of these Pt pellets were low (2.74 to 2.96 g/cm³ for anapplied compaction pressure of about 0.6 to 1 MPa and 3.71 to 3.75 g/cm³for an applied compactions pressure of about 2.1 MPa), as compared withthe density of solid Pt (21.45 g/cm³). These densities correspond to avolume void space in the Pt pellets of between 81.6 and 87.2 volumepercent. This high volume fraction of void space and the correspondinghigh gravimetric surface area explains the high degree of non-faradaiccharge injection that results for modest applied potential for the Ptelectrode.

FIG. 6 provides cyclic voltammetry measurements (20 mV/sec using 1 Maqueous NaCl electrolyte) for nanostructured platinum electrodes,showing that charging is predominately by double-layer charge injection,which is a non-faradaic process. There are no current peaks due toFaradaic processes and the current at constant voltage scan rate (20mV/sec) varies little with potential. From plots of current versuspotential scan rate in 1 M aqueous NaCl electrolyte), the electrodecapacitance is about 14.5 F/g.

Most importantly, it is found that the nanoporous Pt electrode remainscharged when disconnected from the power source, removed from theelectrolyte, and dried. Indication of this retained charge is providedby reimmersion of the nanoporous Pt electrode in the 1 M NaClelectrolyte, and finding that the electrode potential is substantiallyunchanged. Initial results indicating this stability are shown in FIG.7. Just like the case for the carbon nanotube electrode, the potentialof the negatively charged electrode is less stable than for thepositively charged electrode, as indicated by the results shown in thelower part of this figure.

Since it is possible that some electrolyte is still retained inside thepores of the Pt electrode during the experiment of FIG. 7, thisexperiment was repeated using much longer time periods after the pelletelectrodes have been disconnected from the power source and theelectrolyte was removed from the electrochemical cell. The electrodepotentials (versus Ag/AgCl) before after this two days exposure of theelectrodes to dynamic vacuum pumping were +0.58 V and +0.45 V for thehole-injected electrode and −0.04 V and +0.02 for the electron-injectedelectrode. The potential between the two electrodes changed from theinitial 0.62 V to a final 0.43 V after removal of the electrodes fromthe electrolyte and applying a dynamic vacuum on these electrodes fortwo days.

To further evaluate the stability of charge storage, the time period inwhich the platinum pellets were exposed to dynamic vacuum was extendedto a week. After this, the Pt electrodes were reimmersed in the 1 M NaClelectrolyte to determine their charge state by electrochemical potentialmeasurements (naturally, without applying any external potential). Highcharge storage is again indicated for the positively charged Ptelectrode (indicated by retention of a 0.28 V potential, versus Ag/AgCl,compared with the potential immediately before electrolyte removal of0.33 V). The negatively charged electrode had lower stability, asindicated by a potential change from the initial −0.68 V (before removalof the electrolyte and the week-long process of drying the electrode indynamic vacuum) to a final potential on initial reimmersion into theelectrolyte of −0.32 V.

All patents and publications referenced herein are hereby incorporatedby reference. It will be understood that certain of the above-describedstructures, functions, and operations of the above-described embodimentsare not necessary to practice the present invention and are included inthe description simply for completeness of an exemplary embodiment orembodiments. In addition, it will be understood that specificstructures, functions, and operations set forth in the above-describedreferenced patents and publications can be practiced in conjunction withthe present invention, but they are not essential to its practice. It istherefore to be understood that the invention may be practiced otherwisethan as specifically described without actually departing from thespirit and scope of the present invention as defined by the appendedclaims.

1-200. (canceled)
 201. A device having tunable response that comprises:(a) a nanostructured electrode component C of a first electrochemicalelectrode and an electrode component D of a second electrochemicalelectrode; (b) an ionically conducting material that is substantiallyelectronically non-conducting that connects said first and said secondelectrochemical electrodes; and (c) a means of providing a voltagebetween said first and said second electrochemical electrodes, wherein(i) the electrode component C is not in direct contact with anelectrolyte, (ii) the electrode component C has an achievablecapacitance of above about 0.1 F/g for substantially non-faradaiccharging, (iii) the major volume fraction of C does not functionpredominately as a catalyst, (iv) properties changes of the electrodecomponent C in response to predominately non-faradaically injectedcharge are used to achieve device performance, and (v) the device isoperable for use as an electron emission device, and wherein, (A) thedevice response is at least partially effected by a change in the degreeof non-faradaic charge injection in the electrode component C, and (B)the nanostructured electrode component C comprises a nanofiber.
 202. Adevice having tunable response that comprises: (a) a nanostructuredelectrode component C of a first electrochemical electrode and anelectrode component D of a second electrochemical electrode; (b) anionically conducting material that is substantially electronicallynon-conducting that connects said first and said second electrochemicalelectrodes; and (c) a means of providing a voltage between said firstand said second electrochemical electrodes, wherein (i) the electrodecomponent C is not in direct contact with an electrolyte, (ii) theelectrode component C has an achievable capacitance of above about 0.1F/g for substantially non-faradaic charging, (iii) the major volumefraction of C does not function predominately as a catalyst, (iv)properties changes of the electrode component C in response topredominately non-faradaically injected charge are used to achievedevice performance, and (v) the device is operable for use as anelectron emission device, and wherein, (A) the device response is atleast partially effected by a change in the degree of non-faradaiccharge injection in the electrode component C, and (B) the device isoperable for use as a cold cathode in devices selected from the groupconsisting of a flat panel display, a lighting fixture, an electronmicroscope, a discharge tube for over-voltage protection, andcombinations thereof.
 203. The device of claim 201, whereinelectrochemical charge injection into said nanofiber causes a variationin field emission.
 204. The device of claim 203, wherein said nanofiberis a nanotube that is opened in a region selected from the groupconsisting of an end region, a side wall region, and combinationsthereof.
 205. The device of claim 204, wherein the nanotube iselectronically contacted with an electronically conducting material thatis intercalated with ions, and wherein the open region is located in amaterial comprising mobile ions that are sufficiently small in size soas be capable of fitting inside said nanotube.
 206. The device of claim204, wherein the nanotube changes field emission properties as a resultof electrochemically inserting ions inside the volume of said nanotube.207. The device of claim 206, wherein the ions electrochemicallyinserted in the nanotube are at least 10 times more numerous than thoseelectrochemically deposited on the exterior surface of said nanotube.